How do I invoice clients who require a PO number in the US?

Understanding why a PO number matters

A purchase order (PO) number is more than a random string of digits your client asks you to type onto an invoice. In many U.S. organizations—especially larger companies, government agencies, universities, healthcare systems, and manufacturers—the PO number is the key that unlocks their entire procure-to-pay process. Without it, your invoice may be delayed, rejected, or routed into a “missing information” queue where it sits until someone has time to investigate. If you invoice clients who require PO numbers, your goal is simple: capture the PO number early, confirm it’s valid, and make sure it appears consistently on every invoice and supporting document the client expects.

From your perspective as a freelancer, agency, consultant, contractor, or small business owner, a PO requirement can feel like extra bureaucracy. But once you understand what the client is doing internally, it gets easier to work with. The PO number typically links to an approved budget, an internal request from a department, and specific payment terms. It may also define what can be billed, when it can be billed, and how the invoice must be formatted. Your invoice is essentially the final trigger that tells their accounts payable team, “This work was delivered under this approved PO—please pay according to the rules.”

If you treat the PO number as a critical invoice field rather than an afterthought, you’ll reduce late payments, avoid email back-and-forth, and build trust with clients who run structured procurement systems.

Purchase order basics in plain language

A purchase order is a document a buyer issues to a vendor to authorize a purchase. It typically includes the buyer’s name and address, the vendor’s name and address, a PO number, a description of goods or services, agreed pricing, quantities, the delivery period, and payment terms. The PO number is what the buyer uses to track the purchase internally. When you send an invoice, the buyer matches it to the PO and, in many organizations, also matches it to evidence of delivery (like a timesheet approval, an acceptance email, a delivery receipt, or a project completion confirmation). This is often called “matching.”

Different organizations use different matching rules:

Some use two-way match: they match the invoice to the PO. Others use three-way match: they match the invoice to the PO and to proof of receipt or acceptance. The stricter the matching process, the more important it is to invoice exactly what the PO authorizes.

In the U.S., PO requirements are common in mid-size and enterprise businesses. Even some small companies enforce PO numbers to keep spend under control. If a client asks for a PO, assume they have a system that will reject invoices without it.

When you should ask for the PO number

The best time to get a PO number is before you start work. The second-best time is before you send your first invoice. The worst time is after you’ve already invoiced and the client’s accounts payable team rejects it.

Here are moments when you should ask:

At the proposal stage: include a line in your proposal that says the client will provide a PO number if required by their company’s process. At contract signing: confirm whether the client’s procurement policy requires a PO before any work begins. Before kickoff: if the client’s project manager says, “We’ll need a PO,” ask for it immediately and confirm what the PO covers. Before billing milestones: if you bill by phases or deliverables, confirm whether each phase has its own PO number or whether one master PO covers the entire project.

Many payment delays come from a simple mismatch: you invoice under “PO pending” while their system requires a valid PO number to route the invoice. Avoid that by building PO collection into your workflow as a standard step.

How to confirm the PO number is valid

Not all PO numbers are created equal. Sometimes the client gives you a temporary reference, a requisition number, a contract number, or an internal project code and calls it a “PO.” Other times they provide a PO that hasn’t been fully approved, or a PO that’s only valid for a specific entity within their corporate structure.

To confirm validity, ask a few practical questions:

Is this the final PO number that accounts payable will require on invoices? Which legal entity is issuing the PO (the exact company name that will pay)? What is the “bill to” address for invoices under this PO? What is the “ship to” or “service location,” if applicable? What is the authorized amount and currency? What are the dates the PO covers (start/end)? Are there any special instructions for invoices (invoice submission portal, email address, file naming rules, required attachments)?

You don’t need to be pushy. You’re protecting both sides: the client wants compliant invoices, and you want to get paid on time. If your client is used to PO workflows, they’ll appreciate that you’re taking it seriously.

PO number requirements you’ll commonly encounter in the US

In the U.S., the most common requirement is simply: “Put the PO number on the invoice.” But some clients are more specific. You might see these expectations:

The PO number must be in a designated “PO Number” field, not buried in notes. The PO number must be printed near the invoice header (top right or near the invoice number). The PO number must match exactly, including leading zeros, hyphens, and prefixes. The “Bill To” must match the purchasing entity named on the PO. Line items on the invoice must match the PO line descriptions (or be close enough for matching). The invoice total must not exceed the remaining PO balance. Taxes and shipping must be billed only if the PO allows them. Payment terms must align with the PO (e.g., Net 30, Net 45, Net 60). The invoice must include the requester’s name, department, cost center, or project code. The invoice must be submitted through a vendor portal or as a PDF to a specific AP inbox. Some clients require you to reference the PO number in the email subject line as well.

Once you know the client’s rules, you can standardize your invoices to meet them.

Step-by-step: invoicing a client who requires a PO number

The easiest way to stay compliant is to follow a repeatable process. Here’s a practical step-by-step flow you can use for U.S. clients:

Step 1: Align on scope and billing structure

Start with a clear agreement on what you are billing for. Are you charging hourly, by milestone, or a fixed monthly retainer? PO matching works best when the invoice line items closely reflect the PO’s line items or at least the PO’s approved description. If your agreement is vague (“Consulting services”), the PO may be equally vague, which is fine. But if the PO is specific (“Website redesign phase 1”), then your invoice should reflect that same language.

If you anticipate changes or add-ons, discuss how they will be handled under the PO process. Many organizations require a PO change order before they can pay for work outside the original authorization. That means you should avoid doing extra work “on goodwill” without confirming whether the PO can be updated.

Step 2: Get the PO number and supporting details

Collect the PO number, the purchasing entity’s legal name, billing address, and any required invoice submission method. Confirm whether the PO is for a single invoice or multiple invoices over time. Confirm the PO cap amount and whether your invoices must track remaining balance.

If the client uses a vendor onboarding process, complete it early. It may include tax forms (commonly W-9 in the U.S.), banking details for ACH, and vendor profile setup in their system.

Step 3: Set up the client profile and PO details in your invoicing system

Use an invoicing tool that lets you store a PO number per invoice and, ideally, per project or per customer so you can reuse it accurately. In invoice24, you can create a client profile, save billing details, and generate professional invoices that include a dedicated PO number field. This matters because AP teams often scan or auto-import invoice data. A clearly labeled PO field makes your invoice easier to process.

Even if your client changes POs frequently, saving the PO number at the invoice level ensures each invoice is properly tagged and traceable.

Step 4: Build the invoice with matching-friendly line items

When you create the invoice, use a clean structure. Include the service period (for example, “Services provided Jan 1–Jan 31”), the project name, and clear line descriptions. If you have an hourly arrangement, consider including hours worked, hourly rate, and the date range. If you have a deliverable-based arrangement, list the deliverable exactly as it appears on the PO or statement of work.

Then add the PO number prominently in the invoice header or a labeled field. Many clients prefer it near the invoice number and invoice date so it is visible at a glance.

Step 5: Add required references and attachments

Some clients require attachments such as timesheets, signed approvals, expense receipts, or proof of delivery. Keep attachments organized and consistent. If the client uses three-way match and expects an acceptance record, include a short note in the invoice or email referencing where acceptance was documented (for example, “Acceptance confirmed by email on Jan 28”).

If you bill expenses, ensure the PO allows expenses and specify whether you are charging them as reimbursable costs. Avoid lumping expenses into a single line item if the client’s policy requires detail.

Step 6: Submit the invoice via the client’s required channel

Some clients accept invoices by email; others require a vendor portal upload. Follow their rules. If the client requires the PO number in the email subject line, do it. If they require a specific PDF naming convention, follow it. These small details can be the difference between same-week processing and a month-long delay.

When emailing, keep your message short and functional. Include invoice number, invoice amount, PO number, and service period in the email body. Attach the PDF invoice and any required backup.

Step 7: Track status and follow up intelligently

After submission, track the invoice’s due date based on the agreed payment terms. If payment is late, follow up with the project owner and AP team, and include the PO number and invoice number in your message. Many AP teams can locate your invoice instantly if they have the PO number; without it, they may have to search manually or ask you for it again.

Invoice24 helps you keep invoices organized, so you can quickly reference what was sent, when it was sent, and which PO number was included.

Where exactly should the PO number appear on the invoice?

Clients often say “Put the PO on the invoice,” but they don’t always specify where. The safest practice is to place it in a clearly labeled field near the top of the invoice, close to the invoice number and invoice date. That’s where AP teams expect to find it, and it’s where automated extraction tools most reliably capture it.

A good structure is:

Invoice Number, Invoice Date, Due Date, PO Number, Client Account/Vendor ID (if applicable)

You can also repeat it in the notes section if the client asks for redundancy, but don’t rely on notes alone. If your invoicing software supports a dedicated PO field, use it every time.

How to handle multiple POs for one client

Some clients issue one PO per project. Others issue one PO per department, per quarter, or per service category. And some clients will issue a master PO plus “release” numbers for each request. It’s common to have multiple active POs with the same client at the same time.

To manage this cleanly:

Create a simple internal naming convention that ties each PO to a project or billing stream. For example: “ClientName – Marketing Retainer – PO 104928.” When invoicing, confirm which PO applies to the specific work period or deliverable. If your invoice covers multiple workstreams and each has a different PO, you may need to split the invoice into separate invoices—one per PO. Many AP systems cannot pay a single invoice that references multiple POs, or they will require manual handling that delays payment.

If you are working across multiple departments, ask whether each department has a separate PO. If so, create separate invoices per department. It’s extra admin, but it dramatically reduces delays.

What to do if the PO number arrives late

Sometimes you do the work, deliver it, and then the client says, “We’re still waiting on the PO.” This happens when internal approvals take longer than expected. In that situation, you have a few options:

Send a “draft invoice” marked as pending PO approval (only if the client requests it and their policy allows it). Hold the official invoice until you receive the PO number, then issue the final invoice immediately. If you must invoice by a deadline (for month-end close), ask the client whether they can provide a temporary authorization reference that AP will accept—many can’t, but it’s worth asking. If the PO delay is recurring, update your contract terms to require PO issuance before work begins or before the next billing cycle.

In general, avoid submitting an invoice without the PO number when the client’s policy requires it. It can trigger rejections and confusion that slow things down more than waiting a few extra days for the PO.

Common mistakes that cause PO-related invoice rejections

PO requirements are strict because buyers are trying to control spending. The good news is that most rejections are avoidable. Here are frequent issues to watch for:

Typing the PO number incorrectly (missing a digit, leaving out a prefix, removing leading zeros). Using the wrong PO (old PO from a previous project, or a PO tied to a different department). Invoicing more than the PO allows (even by a small amount). Invoicing for items not listed or not authorized on the PO. Billing outside the PO date range. Submitting to the wrong AP email or wrong portal. Missing required attachments (timesheets, acceptance, receipts). Mismatched “Bill To” entity (client’s subsidiary vs parent company). Incorrect vendor name on the invoice compared to what they have on file. Wrong tax handling (charging sales tax when not applicable, or failing to itemize tax when required).

When you treat invoicing as part of delivery, rather than an afterthought, these problems drop quickly.

How to invoice against a PO balance over time

If your client issues a PO for a total amount and expects you to invoice against it over multiple billing cycles, you need to manage the PO balance carefully. Many AP systems will automatically stop payment if your invoice would cause the PO to exceed its authorized amount. Even if the overage is accidental, the system may block payment until the PO is amended.

Practical tips:

Ask for the PO’s authorized amount and track your billed totals against it. If you see you’re approaching the cap, warn the client early and ask whether they will increase the PO or issue a new one. Keep line items consistent so the client can match and reconcile quickly. If a PO is nearly exhausted, consider billing smaller increments to avoid a final invoice that inadvertently pushes over the limit due to rounding, tax, or approved expenses.

Invoice24 makes it easy to see what you billed previously to the same client and keep your invoice history organized, which helps you monitor spend against the PO.

Retainers, subscriptions, and recurring invoices with PO numbers

Recurring billing adds an extra layer to PO workflows. Some clients issue an annual PO for a monthly retainer; others issue quarterly POs; some require a new PO every month. Don’t assume the client’s approach—confirm it.

Best practices for recurring invoices:

Include the service period on every invoice (for example, “Monthly retainer for February 2026”). Use the same PO number consistently if the PO covers the entire term. If the client issues new POs periodically, update the PO number on the next invoice before it is sent. Ask the client to notify you before a PO expires or is replaced. If the client changes POs mid-cycle, clarify which portion belongs to which PO and split invoices if necessary.

With invoice24, you can set up recurring invoices and ensure the PO number is included properly each cycle, reducing the chance of missed fields.

How to invoice government or public-sector clients requiring PO numbers

If you work with U.S. government agencies, municipalities, public schools, or public universities, PO compliance often becomes non-negotiable. These entities usually have strict procurement rules and sometimes require additional vendor information. They may also require specific wording, contract references, or invoice routing codes.

While every agency is different, you can expect:

Mandatory PO number on the invoice. Detailed line items tied to authorized scope. Strict invoice submission channels. Longer payment terms or scheduled payment cycles. Documentation requirements (deliverable acceptance, service dates, supporting backup).

In these cases, build a checklist and follow it every time. It’s also wise to ask whether they have an “invoice submission guide” or vendor handbook. If they do, follow it precisely.

Sales tax, exemptions, and PO-based invoicing

Sales tax in the U.S. is complex because it varies by state and sometimes by local jurisdiction. Whether you charge sales tax depends on what you are selling, where the sale is sourced, and whether the buyer is tax-exempt. Some organizations will provide an exemption certificate; others will not.

When a client issues a PO, it does not automatically mean tax is handled. You still need to follow applicable tax rules. From a workflow standpoint:

If you are charging sales tax, itemize it clearly on the invoice. If the client claims exemption, request the proper exemption documentation and keep it on file. If the PO specifies tax treatment, align your invoice to it, but don’t rely on it as your only guidance.

If you’re unsure about your tax obligations, consider speaking with a qualified tax professional who understands your state and service type.

Payment terms: what to expect and how to protect cash flow

Clients that require POs often have standardized payment terms like Net 30, Net 45, or Net 60. Sometimes terms start from the invoice date; other times they start from the date the invoice is approved or entered into their system. That difference matters. An invoice dated January 1 could be “entered” on January 10, which may push payment out by another week or more depending on their process.

To protect cash flow:

Invoice promptly as soon as the billing milestone is reached. Submit using the correct channel so the invoice gets entered quickly. Ask whether payment terms start from invoice date or approval date. Include all required data to avoid rejections that restart the clock. For large projects, use milestone billing rather than waiting until the end.

Invoice24 helps you issue invoices quickly with consistent formatting, which makes it easier for clients to process them without delay.

What to do if the client refuses to pay because the PO is missing

If a client tells you they cannot pay without a PO, treat it as a process issue, not a personal conflict. In most structured organizations, AP literally cannot process a payment that isn’t linked to an approved PO, even if the project manager wants to pay you.

Here’s how to handle it:

Ask the client’s project owner to initiate the PO request or amendment immediately. Provide the documentation they need to generate the PO (scope summary, pricing, service dates, your vendor info). Reissue the invoice with the correct PO number once received. If the client already received the invoice, send a revised version referencing the same invoice number if their policy permits, or issue a corrected invoice with a new number if required. Keep a clear paper trail so there’s no confusion about what’s being billed.

If this becomes a recurring pattern, update your onboarding checklist and contract terms so the client is responsible for providing a PO before work begins or before each billing cycle.

How to write invoice notes that support PO processing

Notes are not a substitute for the PO field, but they can help reduce confusion. Good invoice notes are short, specific, and useful for matching. Consider including:

The service period covered by the invoice. The project or statement-of-work reference. The name of the internal requester or project owner. A brief confirmation of delivery or milestone completion. Any approved change order reference if scope changed.

Avoid overly long notes. AP teams want clarity, not a narrative.

Handling change orders and PO amendments

One of the most common PO-related issues is extra work that wasn’t included in the original authorization. In PO-driven organizations, “Yes, go ahead” from a project manager is not always enough to get paid. The buyer may need to amend the PO (increase the amount, add a new line item, extend dates) before AP can pay you.

To manage this:

When scope changes, send a short written change summary with pricing and ask whether the PO will be amended. If the client needs internal approvals, pause the new work until you have confirmation. Keep change orders separate from the original scope so it’s easy to invoice them cleanly. When invoicing, reference the updated PO number or PO revision if the client uses them.

This approach keeps relationships healthy and reduces the risk of disputes later.

How invoice24 fits into PO-based invoicing

PO requirements become easy when your invoicing system supports structured invoice data and consistent formatting. Invoice24 is designed to handle real-world client requirements, including PO numbers, recurring invoices, clear line items, and professional PDF invoices that look clean and readable for both humans and AP systems.

Here’s how to use invoice24 effectively for PO clients:

Create a client profile with accurate billing details so every invoice is consistent. Include the PO number in a dedicated field so it’s prominently displayed. Use itemized line items that match the PO language and service period. Generate a polished invoice PDF for email or portal upload. Maintain invoice history so you can track what was billed under which PO and follow up quickly if needed.

Even if you have many clients with different requirements, a consistent workflow inside invoice24 reduces mistakes and makes it easier to scale.

A simple PO invoicing checklist you can reuse

Before sending the invoice, run through this checklist:

Confirm the PO number is correct and formatted exactly as provided. Confirm the invoice “Bill To” matches the purchasing entity on the PO. Confirm line items reflect the authorized scope and descriptions. Confirm service dates are within the PO coverage period. Confirm total does not exceed remaining PO balance. Confirm required attachments are included (timesheets, receipts, acceptance proof). Confirm submission method (email vs portal) and any subject line or naming rules. Confirm payment terms and due date are correct. Export a clean PDF and keep a copy for your records.

This checklist may feel like extra work at first, but it quickly becomes second nature—and it dramatically reduces delayed payments.

Frequently asked questions about invoicing with PO numbers

Do I need a PO number for every invoice? Not always. Some clients require POs only for certain departments or purchases above a threshold. If the client says they require a PO, assume every invoice tied to that work needs one unless they explicitly say otherwise.

Can I put multiple PO numbers on one invoice? Some companies allow it, but many AP systems do not. The safest approach is one invoice per PO. If the client wants one invoice, ask their AP team whether multiple POs are acceptable and how they want it formatted.

What if the PO number changes mid-project? Ask the client which invoices should reference the old PO and which should reference the new one. Often the new PO applies going forward, while prior work remains under the original PO. If needed, split invoices by date range.

What if I accidentally invoiced without the PO number? Send a corrected invoice immediately. Include the PO number prominently and follow the client’s preferred correction process. Some organizations prefer a revised invoice with the same invoice number; others require a credit note and a new invoice. Ask their AP team if you’re unsure.

Is a PO number the same as a contract number? Not necessarily. Some clients use both. A contract number may govern the overall agreement, while a PO number authorizes specific spend. If the client has both, include both where appropriate, but never substitute a contract number for a required PO number.

Putting it all together

Invoicing clients who require a PO number in the U.S. is mainly about discipline and consistency. Get the PO number early, confirm it’s valid, make sure your invoice matches the PO’s authorized scope, and place the PO number in a clear, dedicated spot on the invoice. Submit through the correct channel with any required attachments and references. If you do those things, you’ll avoid the most common AP rejections and get paid faster.

With invoice24, you can manage PO-based invoicing smoothly by keeping client details organized, adding PO numbers to invoices properly, and producing professional invoices that are easy for accounts payable teams to process. The result is fewer delays, fewer emails, and a cleaner, more predictable payment workflow—exactly what you want when you’re growing your business.