How do I invoice clients who pay via online payment portals in the US?

Understanding online payment portals and why invoicing still matters

Online payment portals are now one of the most common ways US clients pay for services and digital products. Your client might prefer to pay through a portal because it feels familiar (think customer dashboards, “Pay Now” pages, saved payment methods, or vendor portals used by larger companies). Sometimes the portal belongs to the client (like a procurement or accounts payable system). Other times it belongs to a payment provider that the client trusts. Either way, you still need to invoice properly. The portal is simply a payment channel, not a complete invoicing system for your business records.

Invoicing is how you clearly document what you delivered, what you’re charging, and what terms apply. It also helps the client’s accounting team match what they paid to what they purchased. Even if a portal collects payment instantly, your invoice is still the “source of truth” for itemized services, dates, tax details (if applicable), and your legal business information. Done well, it reduces disputes, speeds up approvals, and keeps your books clean.

Invoice24 is designed for exactly this reality: clients pay in different ways, but you want one consistent invoicing process. You create professional invoices, keep numbering and records organized, provide clear payment instructions, and track statuses—whether payment happens through a portal link you share or through the client’s own vendor system.

Start with the basics: what counts as an invoice in the US

In the US, an invoice is typically a business document that requests payment and describes the transaction. Requirements vary depending on your industry and state, but generally, a professional invoice should include:

1) Your business name (and “doing business as” name if relevant), address, and contact information.
2) The client’s name and billing address (and sometimes the department or contact person).
3) A unique invoice number and issue date.
4) A description of what you provided, with quantity, rate, and totals.
5) Payment terms (for example: Due on receipt, Net 15, Net 30).
6) The amount due, currency (USD), and any applicable taxes or fees.
7) The payment method(s) and instructions (including portal instructions if necessary).

If your client pays through an online payment portal, you can still include “Pay Now” instructions or a portal reference, but you should keep the invoice itself complete and readable. Think of the portal as a way to settle the invoice, not as the invoice.

Two common portal scenarios and how to invoice for each

Before you choose the best approach, figure out which portal type you’re dealing with. Most situations fit into one of these categories:

Scenario A: You send the invoice, the client pays through a portal link

This is the simpler workflow. You generate and send the invoice yourself, then include an online payment option so the client can pay by card or bank transfer through a portal. The portal could be your invoice’s payment page, a secure checkout link, or a hosted payment page. In this case:

- Your invoice number is the key identifier.
- The portal payment should reference that invoice number.
- Your records should show “Sent,” “Viewed,” “Paid,” and the date/time paid.

Invoice24 supports streamlined invoicing with online payment-friendly layouts, clear totals, and the ability to communicate payment instructions cleanly.

Scenario B: The client requires you to use their vendor portal

Larger clients often require vendors to submit invoices through a portal (sometimes called an AP portal, vendor portal, procurement portal, or supplier portal). Here, the client might reject email invoices entirely. In this case:

- You still create your invoice in Invoice24 for your own records and numbering consistency.
- Then you upload or manually enter the invoice into the client’s portal.
- The portal may generate its own “submission ID” or “voucher number.”

The goal is to keep your invoice number consistent everywhere and store the portal’s submission ID as a reference. That way, if the client asks, “What’s the portal reference?” you can answer instantly without losing your internal audit trail.

Design your invoice so it matches portal payment and accounting needs

When clients pay via portals, the biggest risk is mismatch: the client pays, but the payment is hard to match to the correct invoice, project, or time period. You prevent that by designing invoices that are easy for both humans and accounting systems to interpret.

Use a clear and consistent invoice numbering system

Choose a numbering system that is unique and sequential. Many businesses use a format like 2026-001, 2026-002, and so on. Others use project-based prefixes (for example: ACME-2026-01). The key is consistency. If your client pays through a portal, they may copy/paste the invoice number into the payment memo field, so avoid special characters that portals might strip out.

Invoice24 helps you keep invoice numbers organized so you don’t accidentally duplicate numbers or break your sequence.

Make line items specific enough to survive internal approvals

Clients paying through portals often have approval workflows: a manager approves the cost, then accounting releases payment. If your invoice only says “Services,” it may get held up. Better line items include:

- The service category (design, development, consulting, marketing, etc.)
- The time period or milestone (“January 1–15, 2026” or “Phase 2 milestone”)
- Quantity and rate (hours and hourly rate, or fixed-price milestone amounts)
- Purchase order or contract reference (if the client requires it)

For example, “Website maintenance — January 2026 (retainer)” is clearer than “Monthly services.”

Add the right identifiers: PO numbers, job codes, and vendor IDs

Many portal-driven payments depend on identifiers. A client might not pay you unless the invoice includes a PO number. Or they might require a vendor ID, project code, cost center, or engagement number. If your client provided any of these, place them in a prominent area near the invoice header—somewhere the accounts payable team can spot quickly.

If the portal requires a PO, it’s best practice to confirm it before you start work. If you’re already delivering and you don’t have a PO, that can turn into weeks of delay. On your side, Invoice24 makes it easy to include these details as part of the invoice header or notes so you don’t miss them.

Choose payment terms that align with portal reality

Clients who pay through portals often run payments on schedules (weekly, bi-weekly, or specific days of the month). If you use Net 15 but the client only issues payments twice per month, your invoice might be “late” in theory but normal in practice. In the US, common terms include:

- Due on receipt: often used for small projects or first-time clients.
- Net 15: popular with small-to-mid clients when trust is established.
- Net 30: very common for B2B relationships.
- Net 45 or Net 60: more common with large enterprises and government-like procurement structures.

If the portal is the client’s system, be realistic and use terms that match the contract. If you want faster payment, consider offering incentives for early payment or requiring partial upfront deposits before work begins.

How to invoice when the portal payment includes processing fees

A common question is whether you can pass portal payment fees (like card processing fees) to the client. The answer depends on your agreement, client relationship, and in some cases state rules or card network rules. Rather than guessing, handle it transparently:

- If your contract says you pass fees to the client, include a separate line item like “Payment processing fee” or “Convenience fee,” and keep it clearly calculated.
- If you don’t pass fees, treat it as a cost of doing business and price accordingly.
- If the client pays via ACH/bank transfer through a portal, fees may be lower or sometimes nonexistent, so you can encourage that method.

Whichever option you choose, make sure the invoice total matches what the client will see in the portal checkout. Surprises are a top cause of failed payments or disputes.

Handling sales tax and compliance when clients pay online in the US

Sales tax in the US can be complicated because it varies by state and sometimes by city or county. Whether you need to charge sales tax depends on what you sell, where you have nexus, and where your client is located and uses the product or service. Many services are not taxed in many states, but some are. Digital products and SaaS can be taxed in some states. It’s important to treat tax as a business compliance topic, not a portal topic.

From an invoicing perspective, if tax applies to your transaction:

- Show the taxable subtotal clearly.
- Show the tax rate and tax amount.
- Show the total due.

If tax does not apply, you can still reduce confusion by including a short note like “No sales tax charged” or “Sales tax not applicable” when clients are used to seeing tax on invoices. The goal is clarity for accounting teams.

Make portal payments easy: include the right instructions without clutter

One of the biggest mistakes is adding too many instructions. A clean invoice is more likely to be approved and paid quickly. You want payment instructions that are specific, but short.

If you provide your own portal link

Include one clear call-to-action, such as: “Pay online using the secure payment link” and ensure the invoice number is included automatically in the payment flow. If the client needs to manually enter something, instruct them to use the invoice number as the reference.

If the client uses their vendor portal

Your emailed invoice (or PDF) should still include a short note like: “Submitted via [Client Name] vendor portal on [date].” If you don’t know the submission date yet, you can add it after submission in your records. Also include a contact email so accounts payable can reach you if they have portal questions.

When to send an invoice vs. when to submit it in a portal

Timing matters. If you email an invoice and then also submit it in the client’s portal, you can create confusion or duplicates. The best practice is:

- If the client requires portal submission: submit through the portal first, then email the client contact a short message confirming submission (unless the client says not to email).
- If the client pays via your payment link: email the invoice with the payment link and confirm receipt if the amount is large.

For recurring work, establish a routine date (for example: “Invoices go out on the 1st of each month”) so the client expects them.

Recordkeeping: reconcile portal payments to invoices without headaches

When a client pays through a portal, your bookkeeping must reflect the payment accurately. The main goal is to connect:

- The invoice (what you billed).
- The payment (what you received).
- The fee (if any).
- The net deposit (what arrived in your bank).

Step-by-step reconciliation workflow

1) Issue the invoice in Invoice24 with a unique number and clear line items.
2) If using a portal link you provide, ensure the invoice number appears in the payment reference.
3) When payment arrives, mark the invoice as paid and record the payment date.
4) If the portal deducted a fee, record the fee separately as an expense so your revenue matches the invoice total and your net deposit matches the bank statement.
5) Save or export the payment confirmation for audit trails, especially for larger invoices.

This workflow helps you avoid a common bookkeeping problem: recording the net deposit as revenue and forgetting fees, which can throw off your income reporting and make month-end reconciliation harder.

Partial payments, retainers, and milestone billing through portals

Portals handle partial and milestone payments differently depending on the provider or client system. Your invoice strategy should reflect what’s contractually agreed.

Partial payments

If the client pays in parts, your invoice should show the original total, the amount paid, and the remaining balance. Some businesses issue one invoice and record partial payments against it. Others issue separate invoices for each installment. Either approach can work, but the best approach is the one that matches how your client’s portal and accounting team prefer to process payments.

Retainers and deposits

For retainers, it’s often helpful to invoice the retainer upfront, receive payment, and then apply it to future work. If you do this, keep your documentation clear. You can use invoice notes like “Retainer to be applied to future services” and track when the retainer is drawn down.

Milestone billing

For fixed-price projects, milestone invoices are ideal for portal payments because they’re easy to approve: “Milestone 1 completed” is straightforward. Define milestones in your contract, then invoice each milestone promptly when it’s accepted.

Handling client disputes when payment happens through a portal

Disputes happen even with the best clients. Portals sometimes make disputes feel more formal because payment is tied to a system. The key is to prevent disputes with clarity and to respond quickly when they occur.

Prevent disputes with documentation

- Use detailed descriptions and attach supporting documentation when appropriate (like a statement of work summary, timesheet, or delivery confirmation).
- Match invoice line items to contract wording.
- Include dates and milestone acceptance language.

If a dispute happens

First, identify whether the issue is about deliverables, pricing, or portal workflow. Many “disputes” are actually portal mismatches: missing PO number, incorrect vendor name, or wrong address. If the client is unhappy with the work, respond professionally and propose a resolution. If the issue is portal-related, correct the invoice details and resubmit as required.

Keeping your invoice history organized in Invoice24 helps you respond quickly with the exact invoice version, sent date, and notes—without scrambling.

Client portals that require specific invoice formatting

Some vendor portals require invoices to follow certain formatting rules. For example, they may want:

- A strict “Bill To” address matching their accounting entity.
- A specific remittance address or your “Pay To” details.
- PO number in a particular field.
- Itemized lines that match the PO lines exactly.
- A specific invoice file type, often PDF.

If you’re dealing with these requirements, treat them like a checklist. Build a standard template in your invoicing process so every invoice you send to that client includes the exact required data. It’s one of the highest leverage ways to reduce payment delays.

What to do when the portal remits payment without enough information

Sometimes you’ll receive a payment deposit with a generic label, especially if it’s a batch payment or the client uses a payment aggregator. If you can’t tell which invoice was paid, follow this approach:

- Check portal payment history for the payer name, payment amount, and timestamp.
- Match the amount to open invoices (exact or close matches).
- Look for portal reference numbers or memos.
- If still unclear, email the client’s accounts payable contact with the payment details and ask which invoice(s) it covers.

To prevent this going forward, always encourage using invoice numbers in payment references. Many clients will comply if you make it easy and consistent.

Invoicing for recurring subscriptions paid through portals

If you offer ongoing services—maintenance, consulting retainers, marketing management, or subscriptions—portal payments can be a great fit. The key is to make recurring invoices predictable:

- Use the same schedule each period (monthly on the 1st, for example).
- Keep line items consistent so approvals don’t reset each month.
- Clearly state the billing period (“Service period: February 1–29, 2026”).
- Include the previous balance and credits if applicable.

Consistency is what makes portal workflows fast. If every month looks different, the client may need more approvals and manual checks.

How to invoice US clients when you’re outside the US

Many US clients pay international vendors through portals, but they often need the invoice to include additional details. Depending on the client and their finance process, they may ask for:

- Your legal business name and address (as registered in your country).
- Your tax identification information relevant to your jurisdiction (not all vendors have the same documents).
- Bank details for international transfers if they use ACH alternatives or wires.
- A completed vendor onboarding form.

Even if you’re paid through an online portal, you should make sure the invoice includes your accurate business details. That helps the client document the expense correctly and reduces onboarding delays.

Protect yourself: invoice terms that matter when clients pay online

Portals can feel instant and automated, but your terms still matter. Consider including these on invoices (when appropriate and consistent with your contract):

- Late fee policy: a clear rate or flat fee applied after the due date.
- Scope clarity: a note that the invoice covers only the described services for the listed dates or milestones.
- Payment method notes: if you accept multiple methods, specify which are preferred for large amounts.
- Contact for billing questions: an email address monitored regularly.

Keep terms short and readable. If you need longer legal terms, place them in your contract and use the invoice to reference the contract rather than duplicating pages of text.

Practical examples: what to write on invoices for portal payments

Here are examples you can adapt for different portal situations. The goal is to say enough without turning the invoice into a long letter.

Example note for your own payment link

“To pay this invoice, use the secure online payment option and include Invoice #2026-014 as your payment reference.”

Example note for a client’s vendor portal

“Invoice #2026-014 submitted through [Client Name] vendor portal. For questions, contact billing@[yourdomain].com.”

Example note for PO-based clients

“PO #45821 — Please match line items to PO lines for approval.”

Example note for recurring billing

“Billing period: January 2026 — Monthly retainer for website maintenance and support.”

Common mistakes that slow down portal payments

Even experienced freelancers and small businesses lose time to avoidable invoicing mistakes. Here are the ones that most often cause delays when clients pay via portals:

- Missing or incorrect PO number.
- Invoice number mismatch between your invoice and portal submission.
- Using vague line items that require extra approvals.
- Incorrect client legal entity name (the portal may have multiple entities).
- Billing address mismatch (especially with enterprise clients).
- Sending the invoice to the wrong contact when a portal submission is required.
- Not tracking the portal submission ID, leading to back-and-forth with accounts payable.
- Not recording fees separately, causing reconciliation confusion.

Fixing these issues once is good; building a consistent Invoice24 workflow so they never happen again is even better.

A simple Invoice24 workflow for clients who pay via online portals

To keep things smooth across all portal types, use a repeatable workflow. Here’s a practical approach that works for most US clients:

1) Confirm the client’s payment process before billing: Are you emailing invoices, sending a pay link, or submitting through their vendor portal?
2) Collect required identifiers: PO number, vendor ID, project codes, and billing entity details.
3) Create the invoice in Invoice24 with clear line items, dates, terms, and totals.
4) Include portal-friendly payment instructions: short, clear, and centered around the invoice number.
5) Send or submit the invoice based on the client’s required method.
6) Track status: sent/submitted, approved (if you’re informed), paid, and reconciled.
7) Store portal references: submission ID, payment confirmation, and any fee breakdown for your records.

This workflow gives you the best of both worlds: the convenience of portal payments and the control of a clean invoicing system that you own.

Frequently asked questions about invoicing clients who pay through portals

Do I still need to send an invoice if the client pays through a portal?

Yes. Even if the portal collects payment, you should issue an invoice for your records and for the client’s accounting needs. The portal is a payment method; the invoice documents what the payment is for.

Should the invoice show the portal transaction ID?

It can be helpful to record the transaction ID in your internal notes after payment is received. If you need to revise or reissue an invoice, don’t rely on the transaction ID alone—your invoice number should remain the primary identifier.

What if the portal payment amount doesn’t match the invoice total?

First check whether fees were deducted, whether the client made a partial payment, or whether a credit was applied. If it’s still unclear, ask the client’s accounts payable team which invoice(s) the payment was intended to cover.

How do I handle refunds or chargebacks through portal payments?

If you issue a refund, document it clearly in your records and consider issuing a credit note or a revised invoice balance depending on how your accounting process works. For chargebacks, keep documentation of deliverables, communications, and invoice terms so you can respond quickly.

How can I get paid faster if a client uses a vendor portal?

The fastest path is usually compliance: submit invoices exactly as the portal requires, include all identifiers, match line items to PO lines when applicable, and submit on a consistent schedule. You can also ask the client what their payment runs are (for example, twice monthly) so you can time submissions accordingly.

Final checklist for portal-friendly invoicing

Before you send or submit an invoice for a US client paying via an online portal, quickly check:

- Invoice number is unique and formatted consistently.
- Client legal name and billing address match what the portal expects.
- PO number and vendor ID are included if required.
- Line items are detailed, dated, and match contract or PO language.
- Payment terms are clear and realistic.
- Totals are correct and match what the client will see at checkout.
- Portal instructions are short and reference the invoice number.
- You have a process to record portal submission IDs and payment confirmations.

When your invoices are structured for portal workflows, you reduce friction for your client’s accounting team and get paid faster. With Invoice24, you can keep your invoicing consistent and professional while still letting clients pay the way they prefer—through the online portals they already use.