How do I invoice clients for fixed-price projects in the US?

Understanding fixed-price invoicing in the US

Fixed-price projects are simple in concept: you agree to deliver a defined outcome for a set amount, regardless of how many hours it takes. In practice, invoicing for fixed-price work can get messy if the scope isn’t clear, if payments aren’t structured properly, or if the client’s procurement process expects specific formatting. In the United States, many clients also want invoices to match purchase orders, include tax-related details, and follow payment terms that align with their accounting system. The good news is that once you set up a consistent invoicing method, fixed-price billing can be smoother than hourly work—because you’re not constantly justifying time logs.

This guide walks through how to invoice clients for fixed-price projects in a way that is clear, professional, and client-friendly. It covers typical payment structures (upfront deposits, milestones, and final invoices), the line-item formats clients expect, how to handle changes in scope, what to include on invoices for US clients, and how to avoid common disputes. It also explains how to set up a repeatable workflow using invoice24 so you can invoice faster, get paid sooner, and spend less time chasing paperwork.

Start with the agreement: your invoice should mirror your contract

A fixed-price invoice works best when it matches a fixed-price agreement. Before you send any invoice, make sure you have a written project summary that includes the deliverables, the total price, the payment schedule, and the payment terms. Your invoice should not introduce surprises. Instead, it should reinforce what the client already agreed to.

At minimum, your agreement should define:

1) The scope of work (what’s included and what’s not included).

2) The fixed price and what it covers (for example, “includes up to two rounds of revisions”).

3) The payment structure (deposit, milestones, or completion).

4) Due dates and payment terms (Net 15, Net 30, etc.).

5) What happens if the scope changes (change orders, revised estimate, additional fixed fee, or hourly add-ons).

When your invoices map directly to these points, clients are less likely to push back, and their accounting team has fewer reasons to delay approval.

Choose the best payment structure for fixed-price projects

There is no single “correct” way to invoice a fixed-price project. The right structure depends on project size, risk, cash flow needs, and the client’s expectations. Below are the most common approaches in the US.

Option 1: 100% upfront (best for small, low-risk projects)

For small projects—like a logo package, a simple landing page, or a limited consulting deliverable—charging the full fixed price upfront can be reasonable. It reduces risk and eliminates payment chasing later. Many clients will accept it if the amount is modest and the delivery timeline is short.

To make this easier for the client, present the invoice clearly as a “Project fee” or “Fixed-price project fee” and include the project name and deliverable summary. If the client is hesitant, offer a smaller deposit structure instead.

Option 2: Deposit + final invoice (a common standard)

A typical setup is 30%–50% upfront and the remainder on completion. This balances your cash flow with the client’s desire to pay after results are delivered. Invoices under this structure often come in two parts:

1) Deposit invoice: “Deposit to initiate project” or “Project kickoff deposit (50%).”

2) Final invoice: “Remaining balance upon delivery.”

This method works well for creative services, software development, marketing retainers with a set deliverable, and most professional services that require time before producing a final output.

Option 3: Milestone-based invoices (best for larger projects)

Milestones reduce risk for both sides. You get paid as you deliver meaningful phases, and the client pays as progress is made. Invoices can align to milestones such as discovery, design, development, testing, launch, or training. The key is to define milestones that are objective and measurable, not vague.

Examples of clear milestones:

- “Discovery completed and requirements document approved.”

- “Design prototype delivered and approved.”

- “Version 1 delivered to staging environment.”

- “Final deliverables delivered and handoff completed.”

Each milestone invoice should clearly reference the milestone name exactly as written in the agreement, along with the milestone amount. This is especially important for US clients that route invoices through procurement systems and require a match against contractual milestones.

Option 4: Progress billing by percentage (use with caution)

Some industries invoice fixed-price projects by percentage complete (for example, “25% complete” or “60% complete”). This is common in construction and some long-running implementation projects. It can work for service providers too, but it requires strong documentation and mutual agreement on what “percentage complete” means. If you use this approach, define a clear method for measuring progress to avoid disputes.

For most freelancers and small teams, milestone billing is easier to justify than percentage billing.

Draft your invoice so it’s easy for US clients to approve

Clients in the US often have internal invoice requirements. Even if your client contact is enthusiastic, the invoice may go to an accounts payable department that follows strict rules. Your goal is to reduce friction by making the invoice complete, clear, and consistent.

A professional fixed-price invoice usually includes:

- Your business name and contact details

- Client name and billing address

- Invoice number (unique and sequential)

- Invoice date

- Payment due date and terms (for example, “Due on receipt,” “Net 15,” or “Net 30”)

- A clear project reference (project name, contract name, or statement of work reference)

- Line items that match the agreement (deposit, milestone, or final fee)

- Total amount due

- Payment instructions (bank transfer, card, ACH, etc.)

- Any relevant purchase order number (PO) if the client requires it

invoice24 can standardize this for you. You can save your business profile, assign the client’s billing details, set default payment terms, and generate consistent invoice numbers so you don’t have to recreate formatting each time.

How to write line items for fixed-price work

The line items on a fixed-price invoice should be descriptive enough to justify the charge, but not so detailed that you box yourself into awkward commitments. The biggest mistake is writing line items that accidentally expand the scope or promise unlimited revisions. You want clarity without overcommitting.

Here are effective formats:

Single line item format (simple projects)

Use one line item when the project is small or when the client prefers minimal detail.

Example line item text:

- “Fixed-price project fee: Website landing page design and build (per agreement dated MM/DD/YYYY)”

If you use this format, include a brief project summary in the invoice notes section so the invoice is self-explanatory.

Milestone line item format (medium to large projects)

Use separate line items for each billing point. This is often the best approach because it shows what the client is paying for at that stage without requiring time-based justification.

Examples:

- “Milestone 1: Discovery and requirements (per SOW)”

- “Milestone 2: Design and prototype approval (per SOW)”

- “Milestone 3: Development delivery (per SOW)”

- “Milestone 4: Launch and handoff (per SOW)”

Deposit line item format (common and client-friendly)

Deposits should be labeled clearly so clients understand they are not being billed twice.

Examples:

- “Project deposit (50%) to initiate work”

- “Deposit applied toward total fixed-price project fee”

On the final invoice, you can show the full project fee and then subtract the deposit as a credit, or simply invoice the remaining balance. Both are acceptable; some clients prefer seeing the full fee and the deposit credit because it reconciles easily.

When and how to invoice: practical timing rules

Timing matters. If you invoice too early without an agreed trigger, clients may delay payment “until something is delivered.” If you invoice too late, you carry the cash flow burden and risk a client going cold. The best timing is tied to objective project events.

Practical timing rules for fixed-price invoicing:

- Send deposit invoices immediately after contract signature and before kickoff.

- Send milestone invoices on the same day the milestone is delivered or approved.

- Send the final invoice on delivery or launch day (or immediately after formal acceptance, if required).

- Avoid “end-of-month batching” unless the client explicitly requests it; it delays payment.

In invoice24, you can save invoice templates for each stage (deposit, milestone, final). That way, sending an invoice becomes a repeatable process rather than a new administrative chore every time.

Include payment terms that help you get paid faster

US clients commonly expect Net 30 terms, especially larger companies. But “common” doesn’t mean “mandatory.” Your payment terms should reflect your risk tolerance and the client relationship. Many freelancers use Net 7 or Net 15 for smaller clients and reserve Net 30 for established businesses or enterprise clients.

Useful payment term strategies:

- For deposits: “Due on receipt” is normal because the deposit triggers project start.

- For milestones: Net 7 or Net 15 can work well, especially if milestones are frequent.

- For final invoices: Net 15 is often a good balance, unless the client has strict Net 30 policies.

You can also specify late payment rules in your agreement and mirror them on the invoice, such as late fees or interest. Even if you rarely enforce it, having it listed can encourage on-time payment.

What about sales tax on fixed-price invoices?

In the US, sales tax rules vary by state and by what you sell. Some services are taxable in some states, and digital products can be treated differently depending on location. Because of this variability, you should not assume sales tax always applies—or never applies. Instead, treat tax as a configuration decision based on your business location, the client’s location, nexus rules, and the type of goods or services provided.

Practical invoicing approach:

- If you are required to charge sales tax, show it as a separate line (or tax field) so the client can clearly see subtotal, tax, and total.

- If you are not charging tax, keep the invoice clean and don’t add confusing tax lines.

- If the client is tax-exempt, keep documentation on file and note “tax-exempt” where appropriate.

invoice24 helps by letting you configure taxes per invoice or per line item, so your fixed-price invoices can be compliant and consistent without manual math.

How to handle reimbursable expenses on a fixed-price project

Fixed-price projects sometimes include reimbursable expenses such as travel, licensing, stock imagery, printing, hosting, or third-party contractor costs. Whether these expenses are included in the fixed fee or billed separately must be clear in your agreement. If you bill expenses separately, your invoice should separate them from the fixed-price fee.

Recommended formatting:

- Keep the fixed-price fee as its own section or line item(s).

- Add expenses as separate line items labeled “Reimbursable expenses” or categorized individually (e.g., “Travel,” “Software license”).

- If the client requires receipts, attach them or provide them through the agreed channel.

Even if the project is fixed-price, reimbursables are often easier for clients to approve when itemized. invoice24 supports adding multiple line items, descriptions, and optional attachments or notes so your invoice stays organized.

Change orders: how to invoice when scope changes

Scope change is the number one reason fixed-price invoices get disputed. Clients may genuinely forget what was included, or they may treat “fixed price” as “anything I think of later is included.” The best way to protect yourself is to treat changes as separate work with separate pricing, documented before you do the extra work.

When scope changes, use one of these approaches:

Approach 1: A separate fixed fee for the change

This is often the cleanest option. You write a short change order that describes the new deliverable, the added fixed price, and the schedule impact. Then you invoice that change as its own line item or even its own invoice.

Example line item text:

- “Change Order #2: Additional user onboarding flow (fixed fee)”

Approach 2: Additional work billed hourly

Some providers keep the core project fixed-price but bill out-of-scope requests hourly. This can work well when the change is uncertain or exploratory. If you do this, keep the hourly work clearly separated from the fixed-price portion and reference the change authorization.

Example line item text:

- “Out-of-scope enhancements per approved request (hourly add-on)”

Approach 3: Re-baseline the project (new fixed price)

If the scope change is large, it may make sense to pause and rewrite the fixed price as a revised total. In that case, you issue a revised agreement and invoice against the new milestones. This is more work administratively, but it can save the relationship if the original scope is no longer relevant.

Whichever approach you use, the invoice should reference the change order or revised agreement so the client’s accounting team understands why the amount changed.

Deposits, retainers, and “prepayments”: invoicing language that avoids confusion

Clients interpret words differently. “Retainer” sometimes implies ongoing availability, not project work. “Deposit” implies a portion of a project price. “Prepayment” can sound like paying for something not yet agreed. Using the right label helps prevent misunderstandings.

For fixed-price projects, “deposit” is usually the clearest term. If you want to communicate that the deposit secures calendar time, you can add a note like “Deposit to reserve project start date.” Just avoid implying that the deposit is non-refundable unless your agreement explicitly states that and you’re comfortable with the legal and relationship implications.

Invoice numbering and record-keeping for US clients

Many US businesses expect invoices to have unique, sequential invoice numbers. This is not just for professionalism; it also helps with bookkeeping and audit trails. A consistent numbering scheme makes it easy to locate invoices, match payments, and reconcile accounts.

Common numbering patterns:

- Year-based: 2026-001, 2026-002, etc.

- Client-based: ACME-001, ACME-002, etc. (useful if you have few clients)

- Hybrid: 2026-ACME-001

invoice24 can generate invoice numbers automatically to reduce errors and prevent duplicates, which is especially helpful when you’re sending multiple milestone invoices across several projects.

Purchase orders (POs): how to invoice when the client requires one

Many US companies require a purchase order number before they will pay an invoice. If a client mentions procurement, “vendor onboarding,” or “AP requirements,” assume a PO might be required. Without a PO, invoices can sit unpaid—not because the client doesn’t want to pay, but because their system literally won’t process it.

Best practices:

- Ask for the PO number before you start work (or at least before you send the first invoice).

- Include the PO number prominently on each invoice.

- Make sure the invoice total and description match what the PO covers.

If the PO amount is lower than your project price, do not invoice anyway and hope it works. Ask the client to update the PO. Mismatches can delay payment for weeks.

W-9 forms and 1099 considerations: what clients may ask for

US clients often request a W-9 form from contractors and freelancers. This helps them prepare year-end reporting (commonly a 1099-NEC for payments to non-employees, depending on the situation). Invoicing is not the same as tax reporting, but being prepared for W-9 requests makes onboarding smoother.

Practical workflow:

- Keep a completed W-9 ready for clients who request it.

- Make sure the name and taxpayer identification details you provide align with your business structure and payment account name.

- Keep your invoices consistent with your W-9 business name to reduce confusion in the client’s accounting system.

Many clients won’t ask for this upfront, but when they do, delays often happen because the contractor scrambles to provide documents. Having a repeatable invoicing and documentation flow in invoice24 helps you look professional and get through AP faster.

How to present “deliverables” without turning the invoice into a scope trap

Clients like clarity, but too much detail can backfire. If you write “includes unlimited revisions” on your invoice, it may become the new expectation—even if your agreement said otherwise. If you list every micro-task, the client may treat it as a checklist and argue that payment is conditional on each item. The safest approach is to reference the agreement and use short descriptions that confirm the billing stage.

Smart wording principles:

- Reference the agreement: “per SOW,” “per contract,” or “per approved proposal.”

- Name the deliverable stage rather than every task.

- Mention revision limits only if necessary, and if you do, keep it consistent with the agreement.

- Avoid ambiguous phrases like “complete project” unless you are truly at final delivery.

Payment methods: make it easy for clients to pay

Even a perfect invoice can be delayed if payment is inconvenient. US clients commonly pay via ACH transfer, check, or credit card, depending on company size and policies. Smaller clients may prefer cards; larger clients often prefer ACH or checks.

To reduce payment friction:

- Offer at least two payment options if possible.

- Put payment instructions directly on the invoice (not buried in an email thread).

- Ensure the “amount due” and “due date” are unmissable.

- Use clear invoice notes like “Thank you—please include invoice number in the payment memo.”

invoice24 supports the practical invoice details clients need, and helps you present a clean, professional payment request without extra back-and-forth.

Late payments: invoicing tactics that reduce chasing

Late payments are common, especially when invoices get stuck in approval queues. The best defense is a combination of clear terms, consistent reminders, and invoices that are easy to approve. You want to avoid sounding aggressive while still being firm.

Effective tactics include:

- Use clear payment terms on every invoice.

- Send the invoice promptly when the billing trigger occurs.

- Include a short, polite note: “Please let me know if your AP team needs anything to process this invoice.”

- Follow up shortly after the due date with the invoice number and amount due.

- For milestone projects, avoid starting the next milestone if the previous one is unpaid (unless you’ve agreed otherwise).

Having your invoices organized in invoice24 means you can quickly reference invoice numbers, dates, and amounts during follow-up without searching through old emails.

Refunds, cancellations, and kill fees: how they affect fixed-price invoicing

Sometimes projects end early—clients change priorities, budgets shrink, or internal stakeholders shift. Fixed-price doesn’t automatically mean you’re paid in full no matter what. The right approach depends on your agreement. Many providers use a cancellation clause or a kill fee to compensate for scheduled time and work already completed.

If a project is canceled:

- Invoice for completed milestones that are already delivered or accepted.

- If your agreement includes a kill fee, invoice it as a separate line item with a clear reference to the clause.

- If you must refund part of a deposit, issue a credit note or document the adjustment clearly so bookkeeping stays clean.

Even when the situation is sensitive, clear documentation protects both sides and keeps the relationship professional.

Practical invoice examples for fixed-price projects

Below are example structures you can adapt. Use them as models for how to format your own invoices in invoice24.

Example A: Deposit + final invoice (simple fixed-price)

Invoice 1 (Deposit):

- Line item: “Project deposit (50%) to initiate work: Brand identity package (per agreement)”

- Amount due: $X

- Terms: Due on receipt

Invoice 2 (Final):

- Line item: “Remaining balance (50%) upon delivery: Brand identity package (per agreement)”

- Amount due: $X

- Terms: Net 15

Example B: Milestone invoicing (larger fixed-price)

Invoice for Milestone 2:

- Line item: “Milestone 2: Design prototype delivered and approved (per SOW)”

- Amount due: $X

- Terms: Net 15

- Note: “PO #12345” (if applicable)

Example C: Fixed price + reimbursables

Invoice:

- Line item: “Milestone 3: Implementation delivery (per SOW)”

- Line item: “Reimbursable expense: Software license (client-approved)”

- Total due: $X

Example D: Scope change with a change order

Invoice:

- Line item: “Milestone 1: Discovery and requirements (per SOW)”

- Line item: “Change Order #1: Additional reporting dashboard (fixed fee)”

- Total due: $X

Common mistakes to avoid with fixed-price invoicing

Even experienced professionals run into invoicing problems. Avoid these common pitfalls to reduce disputes and get paid faster.

1) Invoicing without a clear billing trigger. If the contract doesn’t specify when you invoice, clients may delay payment until they feel satisfied. Tie invoices to deposits, milestones, or delivery events.

2) Using vague descriptions. “Work completed” or “Services rendered” often triggers questions from AP departments. Use specific milestone or project names that match the agreement.

3) Forgetting the PO number. If the client requires a PO, missing it can stall payment entirely.

4) Over-detailing the scope. Your invoice is not a new contract. Avoid adding promises that weren’t in the original agreement.

5) Waiting too long to invoice. Invoicing late trains the client to pay late. Invoice promptly when the agreed trigger happens.

6) Mixing out-of-scope work into the same line item. Keep change orders and add-ons separate so clients can approve them cleanly.

Setting up a fixed-price invoicing workflow in invoice24

If you invoice fixed-price projects regularly, the biggest win is consistency. A repeatable workflow reduces errors, shortens approval time, and makes your business look organized. Here’s a straightforward approach you can implement in invoice24.

Step 1: Create a project-based client profile

Save the client’s legal name, billing address, and any required billing fields (like department name or procurement contact). If they have a PO requirement, record that so you remember to add it to each invoice.

Step 2: Build invoice templates for each billing stage

Create reusable invoice formats for:

- Deposit invoice

- Milestone invoices

- Final invoice

- Change order invoice

Using templates ensures your wording stays consistent and aligned with your agreement.

Step 3: Standardize your line-item naming

Use milestone titles that match your contract language. This reduces confusion and makes it easier for the client to match the invoice to internal approvals.

Step 4: Set default payment terms and late policy language

Choose the payment terms you use most often (for example, Net 15) and apply them consistently. If you include late fee language, keep it short and professional.

Step 5: Send invoices promptly and track what’s outstanding

Fixed-price projects move quickly when billing is handled cleanly. Use invoice24 to keep invoices organized by status—draft, sent, paid, or overdue—so you always know where cash flow stands and which clients need a reminder.

How fixed-price invoicing differs from hourly invoicing

Fixed-price invoices are outcome-based rather than time-based. That means your invoice should communicate value and progress, not effort. With hourly work, clients may scrutinize line-by-line time entries. With fixed price, clients are more likely to focus on whether the deliverable is complete and whether the billing matches the agreed schedule.

This difference is a strength. If you structure milestones well, you can spend less time defending your work and more time delivering it. The invoice becomes a simple confirmation of what has been achieved.

Final checklist before you send a fixed-price invoice

Before sending your invoice, run through this quick checklist:

- Does the invoice match the contract language (project name, milestone name, and amount)?

- Is the invoice number unique and properly formatted?

- Are client billing details correct?

- Is the PO number included if required?

- Are payment terms and due date clearly stated?

- Are taxes applied correctly (if applicable)?

- Are reimbursable expenses separated and clearly labeled (if applicable)?

- Are change orders listed separately with references (if applicable)?

- Are payment instructions included and easy to follow?

If you can say “yes” to these, your invoice is positioned to be approved quickly.

Conclusion: make fixed-price invoicing predictable and stress-free

Invoicing clients for fixed-price projects in the US doesn’t need to be complicated. The winning formula is simple: define scope clearly, choose a payment structure that matches the project risk, invoice promptly at agreed triggers, and format your invoices so they’re easy for accounts payable to approve. Most problems come from mismatches—between the contract and the invoice, between the client’s internal process and your invoice details, or between the original scope and what the client later asks for.