What’s the best way to invoice clients for long-term projects in the US?

How to Invoice Clients for Long-Term Projects in the US

Long-term projects are a different beast than quick, one-off jobs. Cash flow matters more, expectations need to be clearer, and small misunderstandings can snowball over months. Invoicing is the system that keeps all of that organized. Done well, it helps you get paid on time, reduces friction, and reinforces your professionalism. Done poorly, it can create awkward conversations, late payments, and scope disputes that eat into your profit.

So what’s the best way to invoice clients for long-term projects in the US? The most reliable approach is to combine a clear billing structure (milestones, progress billing, retainers, or recurring monthly invoices) with consistent documentation, predictable timing, and payment terms that match the project’s risk and workload. The goal is simple: align when you invoice with when you deliver value—and set both you and your client up for a smooth, repeatable routine.

This guide walks through the most effective invoicing methods for long-term projects, how to choose the right one, how to write invoice terms that actually work, and practical steps you can apply immediately using invoice24.

Why Long-Term Project Invoicing Needs a Different Strategy

In short projects, the invoice is often just a formality: do the work, send the bill, get paid. In long-term projects, invoicing becomes a management tool. It’s how you track deliverables, confirm scope, control cash flow, and keep communication structured.

Long-term work often includes:

• Changing requirements and evolving scope

• Multiple stakeholders and approval steps

• Long payment cycles, especially for larger clients

• Higher financial risk if you wait until the end to bill

• Ongoing expenses you’re paying before you get reimbursed

Because of this, the “invoice at the end” method is rarely the best. Instead, you want a billing plan that protects your time and keeps the client comfortable with a predictable cadence.

The Most Common Invoicing Methods for Long-Term Projects

There isn’t one perfect method for every project, but there are a few that consistently work well in the US. The best choice depends on the type of work, how clearly you can define deliverables, the client’s procurement process, and your tolerance for risk.

1) Upfront Deposit Plus Progress Billing

This is one of the most practical models for freelancers, agencies, and service businesses. You take an upfront deposit to secure the project and cover initial ramp-up, then invoice as the project progresses.

How it typically looks:

• 20%–50% deposit due before work begins

• Remaining amount billed in phases (monthly, biweekly, or at milestones)

Why it works:

• The deposit filters out flaky clients and locks in commitment

• You reduce the risk of doing lots of work without payment

• Clients don’t feel like they’re paying everything before seeing value

Where it fits best:

• Web design and development

• Branding and creative projects

• Consulting engagements with a defined roadmap

• Any project with a clear start phase (discovery, planning, onboarding)

Using invoice24, you can create an initial deposit invoice and then generate subsequent invoices using saved line items, templates, and consistent payment terms. This creates a clean paper trail and makes it easy for clients to understand what they’re paying for at each step.

2) Milestone-Based Invoicing

Milestone invoicing means you bill when you hit defined deliverables. Instead of billing by time, you bill by outcomes. This is often perceived as fair by clients because they pay when tangible progress is delivered.

Example milestone structure:

• Milestone 1: Project kickoff + requirements finalized (invoice due)

• Milestone 2: First draft / prototype delivered (invoice due)

• Milestone 3: Final delivery and handoff (invoice due)

Why it works:

• It aligns payment with clear value

• It keeps both sides accountable to timelines

• It’s easy to explain and defend

Common mistake to avoid:

Don’t make milestones too large or too far apart. If a milestone covers two months of work, you’re still effectively waiting two months to get paid. Smaller milestones reduce risk and keep momentum.

Tips for better milestone invoicing:

• Define deliverables precisely so “done” isn’t subjective

• Include acceptance criteria and a review window

• Clarify what happens if feedback is delayed (it often is)

invoice24 makes milestone invoicing straightforward by letting you duplicate invoice templates and customize the milestone description in the line items so the invoice matches the contract language.

3) Monthly Recurring Invoicing (Fixed Monthly Fee)

If your long-term project is ongoing—like marketing, maintenance, managed services, or advisory work—a fixed monthly fee can be the simplest and most stable option. You invoice on a consistent date each month for a consistent amount.

Why it works:

• Predictable cash flow for you

• Predictable budgeting for your client

• Minimal admin overhead once the system is set

Where it fits best:

• SEO and content marketing

• Social media management

• IT support and maintenance

• Fractional roles (fractional CFO, COO, marketing lead)

To make this model smooth, your scope needs to be well-defined. For example, instead of “marketing support,” define what’s included (number of campaigns, reporting cadence, response times, meeting frequency) and what triggers additional charges.

With invoice24, you can keep recurring invoices consistent with saved client details, payment instructions, and standardized line items, so monthly billing becomes a quick, reliable workflow.

4) Time and Materials (Hourly Billing with Regular Statements)

Some long-term projects can’t be clearly scoped upfront. In those cases, time and materials billing (T&M) is common: you track hours and expenses and invoice the client regularly.

Typical cadence:

• Weekly or biweekly invoices for active projects

• Monthly invoices for slower-moving engagements

Why it works:

• Flexible when scope changes frequently

• Transparent when paired with clear time tracking

• Often standard in professional services and contracting

To make hourly billing client-friendly:

• Provide a summary of work performed by category (e.g., planning, development, QA)

• Keep descriptions specific enough to be meaningful

• Separate billable hours from non-billable items if applicable

One key to success is consistency. If you invoice weekly, invoice weekly. When clients learn your rhythm, they approve and pay faster because it becomes routine rather than an interruption.

5) Retainer Billing (Prepaid Hours or Access)

Retainers are widely used in the US for long-term relationships where the client wants ongoing access to your time. Retainers can be structured as prepaid hours (use-it-or-lose-it or rolling) or as a flat fee for availability and outcomes.

Common retainer styles:

• Prepaid hours: Client pays for a set number of hours each month

• Access retainer: Client pays for priority access and defined services

• Hybrid: A base retainer plus hourly overages

Why it works:

• You get paid upfront, reducing cash flow stress

• Clients get reliable access and predictable support

• It builds a long-term partnership rather than transactional billing

What to clarify in your retainer terms:

• Whether unused hours roll over and for how long

• Overage rates and how they’re approved

• Response times and scheduling rules

• What is included versus out of scope

invoice24 helps retainers feel professional by letting you standardize monthly retainer invoices with consistent descriptions and payment instructions, reducing confusion and back-and-forth.

How to Choose the Best Invoicing Method

Here’s a practical way to pick a method without overthinking it. Match your billing strategy to how the work is defined and how the client experiences value.

If the deliverables are clear

Choose milestone-based invoicing or deposit plus milestones. This works when you can define outputs like “design concepts delivered” or “phase 1 development complete.”

If the work is ongoing and predictable

Choose a fixed monthly invoice or retainer. This works when value is delivered continuously (support, optimization, management).

If scope is uncertain or changes often

Choose time and materials with a steady billing cadence (weekly/biweekly/monthly). This reduces disputes because you’re billing for effort with documentation.

If you need maximum cash flow stability

Choose a retainer or deposit + monthly progress invoices. This is especially helpful if you’re hiring subcontractors or fronting project costs.

The “best” method in practice

For many service providers, the best overall approach is a hybrid: an upfront deposit to start, then milestone or monthly progress invoices for the remainder. It balances trust, momentum, and financial safety.

Key Elements Every Long-Term Project Invoice Should Include

No matter which method you choose, long-term invoices should be detailed enough to prevent disputes and simple enough to be approved quickly. Aim for clarity, not complexity.

1) Clear project and invoice identification

Include:

• Invoice number

• Invoice date

• Due date

• Client name and billing address

• Project name or purchase order (PO) number if applicable

Many US businesses route invoices through accounting systems that rely on invoice numbers and PO references. Missing these can delay payment even if the client wants to pay quickly.

2) A simple summary of what’s being billed

For milestone or progress billing, use line items like:

• “Phase 2 milestone: prototype completion”

• “Monthly project progress billing: January 2026”

For time and materials, summarize by category:

• “Development and implementation: 18.5 hours”

• “Project management: 4 hours”

The goal is for an approver to understand the invoice in under a minute.

3) Payment terms that match reality

Common US payment terms include:

• Due on receipt

• Net 7

• Net 15

• Net 30

• Net 45 / Net 60 (often large organizations)

Choose terms you can live with and that align with the client’s normal process. If a large enterprise can’t do Net 7, forcing it may just slow things down. Instead, adjust pricing and billing cadence to protect yourself.

4) Late fees and collections language (kept calm and professional)

Late fees can be a deterrent, but they’re mainly useful as a signal that payment timing matters. Keep the language professional and consistent with your contract.

Common approach:

• “A late fee of X% per month may be applied to overdue balances.”

Even if you rarely enforce it, including it sets expectations and gives you a tool if invoices slip repeatedly.

5) Change order or out-of-scope handling

Long-term projects almost always change. Your invoice system should support that reality. A small note can prevent big arguments:

• “Out-of-scope requests will be billed separately upon approval.”

If scope expands, add separate line items or a separate invoice so it’s clearly distinct from the original agreement.

Best Practices for Long-Term Project Billing in the US

Invoice on a schedule, not on a feeling

Many payment delays happen because invoicing is inconsistent. If you invoice “whenever you remember,” clients process invoices “whenever they get to it.” Create a predictable cadence: the 1st and 15th, or the last business day of the month, or at milestone acceptance.

Consistency builds trust and reduces approval friction.

Send invoices to the right person the first time

In long-term work, it’s common for your main contact not to be the person who processes invoices. Early in the relationship, confirm:

• Who receives invoices

• Whether invoices need a PO number

• Whether the client needs you in a vendor system

• Whether invoice approvals happen weekly, biweekly, or monthly

Then send invoices accordingly. If an invoice sits in the wrong inbox for two weeks, you’ve essentially given the client extra credit for free.

Use itemization that matches the contract

Invoices should mirror your agreement. If your contract is organized by phases, invoice by phases. If it’s monthly services, invoice by month. When invoices match the contract structure, approval becomes routine and disputes drop.

Keep descriptions professional and specific

“Work on project” is vague. “January progress billing: UX revisions and design system updates” is better. You don’t need a novel—just enough detail that someone reviewing the invoice can connect it to real work.

Offer multiple payment options

In the US, clients may prefer ACH/bank transfer, credit card, or check depending on their accounting setup. When you offer multiple options, you remove excuses and speed up payment. If you accept card payments, be mindful of fees and build them into your pricing where appropriate.

invoice24 is designed to support professional invoicing workflows, so you can present payment instructions clearly and keep client details saved for fast repeat billing.

Use partial payments for large milestones

If a milestone is big, consider splitting it into two invoices: one due at milestone start and one due at milestone completion. Clients often accept this more readily than a huge invoice all at once, and it protects you if the project stalls near the end.

Document approvals and changes in writing

This doesn’t need to be complicated. A simple email confirmation or written approval for a milestone is enough to support your invoicing. The key is having a clear record of what was agreed and when.

Plan for pauses and delays

Long-term projects often pause due to client feedback cycles, stakeholder availability, or budget changes. Your invoicing approach should account for this. Consider including terms like:

• Billing continues during active engagement periods

• If the project is paused for more than X days, a restart fee may apply

• Deliverable timelines shift when approvals are delayed

Even if you keep the language light, having a structure protects both sides from confusion.

How to Handle Expenses, Reimbursements, and Materials

If your project includes reimbursable expenses (travel, software, subcontractors, printing, hosting), you need a clean method for billing them.

Separate expenses from labor

Keep expense line items distinct so clients can review them easily. Example:

• “Software license reimbursement: February tool subscription”

• “Travel reimbursement: onsite meeting (mileage)”

Clarify markup policies upfront

Some providers bill expenses at cost. Others add a handling percentage. Either is fine, but the client must know which one applies before the first expense hits an invoice.

Invoice expenses on the same cadence as labor

Don’t let expenses pile up for months. If you’re billing monthly, include expenses monthly. This prevents sticker shock and keeps your cash flow healthier.

Common Payment Terms and How They Affect Cash Flow

Payment terms are not just a line on an invoice—they shape how stable your business feels during a long project.

Net 30 is common, but it’s not always ideal

Net 30 is widely used in the US, especially with established companies. But “Net 30” can turn into 45 days or more if approvals are slow. If you accept Net 30, protect yourself by invoicing more frequently or collecting a deposit.

Shorter terms work best for smaller clients

For small businesses and direct clients, Net 7 or Net 15 can be reasonable—especially if you provide an easy way to pay. Many will pay quickly if the invoice is clear and the process is simple.

Due on receipt works for retainers and deposits

Deposits and retainers are typically due before work begins or before a new billing cycle starts. This sets a strong expectation that payment enables continued work.

How to Reduce Late Payments on Long-Term Projects

Late payments are often preventable with structure and consistency. Here are the most effective ways to reduce delays without sounding aggressive.

Send invoices promptly

The sooner you invoice after a milestone or billing period, the sooner you get paid. Waiting “until things calm down” usually means you forget details, and the client treats the invoice as less urgent.

Use friendly, consistent reminders

Most late payments aren’t malicious—they’re administrative. A polite reminder before the due date and again shortly after can dramatically improve on-time payment rates.

Make it easy to approve

Approval is the bottleneck for many clients. If your invoice is confusing, it goes to the bottom of the pile. Keep the format consistent, label the billing period, and match your contract language.

Stop work policies (used carefully)

For some projects, it’s appropriate to include a policy that work pauses if invoices become significantly overdue. This is less about punishment and more about preventing unlimited unpaid labor. If you use this approach, keep the wording professional and apply it consistently.

Using invoice24 to Streamline Long-Term Project Invoicing

The best invoicing strategy is the one you can follow consistently. Long-term projects create a lot of admin if you’re building every invoice from scratch. That’s where a dedicated invoicing workflow matters.

With invoice24, you can keep long-term billing organized by:

• Saving client details for repeat invoicing

• Using consistent invoice templates for milestones, retainers, or monthly billing

• Clearly listing line items and descriptions that match your agreement

• Keeping invoice numbering and records orderly for accounting

• Quickly duplicating prior invoices so each billing cycle takes minutes instead of hours

When clients see consistent, professional invoices month after month, they treat your billing as a normal business process—because it is.

Practical Examples of Strong Long-Term Billing Structures

Sometimes the easiest way to decide is to see what a good setup looks like. Here are a few real-world patterns you can adapt.

Example A: Web Development Project (3–5 months)

• 30% deposit due upfront

• 30% due after design approval

• 30% due after beta launch

• 10% due at final handoff

This structure keeps cash flow steady and reduces end-of-project risk.

Example B: Marketing Management (Ongoing)

• Monthly fixed fee invoiced on the 1st of each month

• Add-on charges for extra campaigns billed separately

This keeps the core predictable and isolates scope creep into clearly approved add-ons.

Example C: Consulting Engagement (Unclear scope, high variability)

• Monthly retainer for priority access and baseline hours

• Hourly billing for overages, invoiced biweekly

This protects availability while keeping flexibility for unpredictable workloads.

Example D: Construction/Contracting Style Progress Billing

• Monthly progress invoice based on percent complete

• Detailed statement of work completed during the period

This is common in industries where work spans long timelines and deliverables are continuous.

Important Legal and Tax Considerations (Kept Simple)

Invoicing overlaps with legal and tax practices, but you don’t need to make it complicated. You just need to be consistent and accurate.

Sales tax and services

Sales tax rules vary by state and by the type of service. Some services are taxable in certain states, and some digital goods and subscriptions have special rules. If you’re not sure, treat tax as a separate line item and consult a qualified professional for your specific situation.

1099 considerations

If you’re a contractor working for US clients, clients may request a W-9 and may issue a 1099-NEC depending on how payments are made and the amounts involved. Keep your invoices organized and consistent so your records match what clients report.

Contracts and invoices should agree

Your invoice is not a substitute for a contract. For long-term projects, always have written terms covering scope, billing schedule, payment terms, and change handling. Your invoice should reflect that agreement rather than introduce new surprises.

Communication Scripts That Keep Billing Smooth

A lot of invoicing problems start as communication problems. Setting expectations early reduces friction later. Here are a few examples of wording you can use in emails or in your onboarding materials.

When setting the billing cadence

“For this project, I’ll invoice a 30% deposit to begin, then send progress invoices at each milestone. Each invoice will be due within 15 days.”

When requesting a PO number

“If your accounting team requires a PO number or vendor details to process invoices, please share those now so payments don’t get delayed later.”

When addressing scope changes

“Happy to add that—because it’s outside the original scope, I’ll send a separate line item (or change order) for approval before starting.”

When following up on a late invoice

“Just a quick reminder that invoice #____ was due on ____. If anything needs clarification for approval, I’m happy to help.”

Simple, polite, and process-focused wins.

The Bottom Line: The Best Way to Invoice Long-Term Projects

The best way to invoice clients for long-term projects in the US is to choose a billing structure that matches the nature of the work and protects your cash flow—then execute it consistently with clear documentation and predictable timing.

If you want the most broadly effective approach, use an upfront deposit to establish commitment, then invoice on milestones or a monthly schedule to keep payment aligned with progress. Add clear payment terms, straightforward line items, and a plan for changes. Finally, make invoicing easy for both you and the client by standardizing your process.

When you use invoice24 to keep client details organized, reuse templates, and generate consistent invoices for each phase or billing period, you create a smooth system that helps clients approve quickly and helps you get paid without stress. Long-term projects become more profitable when billing is not an afterthought, but a reliable routine.