How Do You Invoice Clients If You Work with Purchase Orders?

How Purchase Orders Change the Invoicing Game

If you’ve ever worked with larger companies, public sector organizations, or growing teams with layered approvals, you’ve probably heard the phrase: “We need a purchase order before you can invoice us.” A purchase order (PO) is a formal document created by the client that authorizes a specific purchase. It tells you what they’re buying, how much they expect to pay, and often includes internal budgeting details and approval sign-offs. For you, the supplier or freelancer, it’s both a green light and a rulebook.

Invoicing with purchase orders isn’t complicated, but it is strict. A client’s accounts payable team might reject invoices that don’t include the PO number, don’t match the agreed amounts, or don’t align with the PO’s line items. Even a small mismatch can lead to delays, re-approvals, or invoices getting bounced back for corrections. The result is frustrating: you did the work, you sent the invoice, and then you’re stuck waiting because the invoice didn’t follow the PO workflow.

The good news is that once you understand the logic behind POs and set up a consistent process, invoicing becomes smoother than invoicing without them. Purchase orders can actually protect you: they confirm approval, clarify scope, and provide an internal reference your client can’t easily dispute later. The key is handling them properly—especially your invoice formatting, documentation, and matching rules.

This article will walk you through exactly how to invoice clients who use purchase orders, from the moment you receive the PO to the moment you get paid. You’ll also learn how to prevent delays with simple checks, how to handle changes, and how to set up a repeatable workflow using invoice24—your free invoicing app built to handle real-world business processes without the usual clutter or hidden fees.

What a Purchase Order Is (and Why Clients Use Them)

A purchase order is the client’s official authorization to buy your product or service. It’s usually generated by their procurement or finance team and approved internally before it’s sent to you. Think of it as the client’s commitment to pay, provided that you invoice according to the PO terms and deliver what’s described.

Clients use purchase orders for a few main reasons:

Budget control: The PO reserves money in a department budget before the spend happens, so finance can track commitments.

Approval workflow: A PO often indicates that the work has already been approved by the relevant manager, procurement, and finance.

Audit trail: Companies need a clear paper trail that shows what was purchased, from whom, and under what terms.

Standardization: POs help accounts payable process invoices quickly because invoices can be matched against a predefined document.

For you, the important takeaway is this: the PO is not just a reference number. It’s a matching system. Your invoice must align with it. When it does, payment is often faster because your invoice fits neatly into their process. When it doesn’t, delays start.

Before You Start: Get the PO Early and Review It

The best time to avoid PO-related problems is before you begin work. If a client uses purchase orders, ask for the PO as soon as the scope and price are agreed—or at least before you deliver. If you wait until after delivery, you could end up in an awkward situation where the client’s internal process blocks your invoice because the PO wasn’t issued correctly.

When you receive the PO, review it like you would review a contract. Specifically, check these details:

PO number: This must appear on your invoice exactly as shown. Some systems are case-sensitive, and missing digits can lead to rejection.

Vendor name and address: Ensure the PO is issued to the correct legal entity (your business name). If it’s wrong, ask for a corrected PO.

Description and line items: Confirm the PO matches what you’re delivering (services, hours, deliverables, products).

Quantity and unit price: Even small differences can trigger mismatch issues in automated systems.

Tax/VAT rules: Ensure the PO reflects the right tax treatment if applicable in your jurisdiction.

Payment terms: Common terms include Net 30, Net 45, or Net 60. This tells you when payment is due after the invoice date.

Billing instructions: Many POs include special requirements like “Include PO number on all invoices,” “Send invoices to a specific email,” or “Reference cost center.”

If anything doesn’t match your agreement, resolve it before invoicing. A corrected PO issued now is much easier than chasing payment later.

The Standard PO Invoicing Workflow (Step by Step)

Most organizations follow a basic “three-way match” process: they match the purchase order, the invoice, and evidence of delivery (like a receipt, delivery note, or acceptance confirmation). Your goal is to make that match effortless. Here’s the typical workflow you should follow:

Step 1: Confirm the PO covers the work you’re billing

Make sure you’re billing only what the PO authorizes. If the PO is for 20 hours and you worked 25, you need either a PO amendment or a new PO for the extra 5 hours (depending on how the client handles overages). Many systems won’t allow invoices to exceed the PO total.

Step 2: Prepare proof of delivery (if relevant)

For products, this might be a delivery note or shipping confirmation. For services, it might be a signed timesheet, completion email, milestone acceptance, or a statement of work completion. Some clients won’t require this from you, but it’s helpful to have ready if the client requests it.

Step 3: Create the invoice and include the PO number prominently

This is the most important step. Your invoice should include the PO number in a highly visible spot—typically near the invoice number, in the header, or in a designated “PO Number” field. If the client’s accounts payable team is entering invoices into a system, they’ll look for it immediately.

Step 4: Match invoice line items to the PO line items

If the PO lists “Design services – 10 hours @ £50/hour,” try to invoice using the same wording and structure. The closer the match, the less chance the invoice gets flagged. Avoid vague descriptions like “Work completed” when the PO is specific.

Step 5: Send the invoice to the correct destination

PO clients often have dedicated invoice email addresses or portals. Sending your invoice to the wrong person (even a friendly contact) can add weeks of delay.

Step 6: Track status and follow up using the PO reference

When you follow up, always mention your invoice number and the PO number. This helps accounts payable find it quickly.

invoice24 is built to support this workflow without making it feel complicated. You can keep your invoices clean and professional while still meeting the strict requirements PO-driven clients enforce.

Where to Put the PO Number on Your Invoice

Accounts payable teams handle a lot of invoices. They’re often working inside systems that are designed to auto-match invoices to POs. If the PO number is missing, unclear, or buried in the wrong place, your invoice can be rejected automatically. Even if it’s not rejected, it can be delayed while someone emails you to ask for the PO number.

Best practice is to put the PO number in at least one of these places:

Invoice header: Near the invoice number and invoice date. This is the fastest for humans to see.

Dedicated field: A labeled “PO Number” field helps clarity and reduces mistakes.

Invoice description or notes: Useful as a backup, but not ideal as the only location.

With invoice24, you can create invoices that clearly display the purchase order number in a consistent format, helping you avoid the common “missing PO number” rejection. The goal is simple: make it impossible for the client to miss it.

How to Match Your Invoice to the PO Without Headaches

The single biggest reason PO invoices get delayed is mismatch. Your client’s system (or their accounts payable team) is looking for a match between your invoice and their PO. The more you align your invoice format with the PO, the faster you get paid.

Here are practical ways to match cleanly:

Use the same line item labels: If the PO says “Monthly retainer – January,” don’t invoice “Consulting services.” Use “Monthly retainer – January.”

Match quantities and units: Hours, days, units, subscriptions—keep the same measurement type.

Match pricing structure: If the PO uses unit pricing, don’t send a single lump sum (unless the PO is also lump sum). If the PO is milestone-based, invoice by milestone.

Don’t exceed the PO total: Many systems block it. If you need to bill more, handle it as a PO change (more on that below).

Keep discounts and extra fees transparent: Surprise shipping fees, rush charges, or “admin” lines often create disputes. If they were agreed, ensure they are reflected in the PO or approved in writing.

invoice24 makes this easier because you can create clear, itemized invoices quickly and keep your line items consistent across recurring work. Clean invoices aren’t just about aesthetics—they’re about compliance with how your client pays.

How to Invoice Against Partial POs or Multiple Milestones

Not every PO is meant to be invoiced in one go. Many clients issue purchase orders that cover an entire project, and you invoice progressively as work is delivered. In that case, it’s essential to invoice in a way that matches the client’s expectation and keeps the remaining balance obvious.

There are a few common approaches:

Milestone billing

If your project has defined milestones—like “Discovery,” “Prototype,” and “Final Delivery”—your invoice should reference the milestone name and amount exactly as agreed. If the PO includes milestone line items, mirror them.

Time-based billing (weekly/monthly)

If you bill hours or days, include a summary period such as “Services provided from 1 Jan to 31 Jan,” and provide a timesheet if the client requires it. Some clients want timesheets attached. Others want them available on request.

Partial delivery for products

If products are shipped in batches, invoice the delivered quantities and reference delivery notes. The invoice should clearly reflect what has shipped and what remains.

In all cases, reference the same PO number and keep the invoice structure consistent. invoice24 helps you produce professional invoices for each stage and keep a record of what was billed and when—so you can quickly respond if the client asks for clarification or history.

Handling PO Changes, Scope Creep, and Overages

Real projects change. The client asks for more work. The scope expands. A timeline shifts. With purchase orders, changes must be handled carefully because the PO is the client’s internal permission to spend. If you invoice beyond what the PO authorizes, your invoice may be rejected—even if the client verbally agreed.

Here’s how to handle changes without risking your payment:

1) Identify when the PO no longer matches the work

If the PO authorizes 40 hours and you’re approaching 35, that’s your trigger to check in. If the client is likely to request more, raise the PO issue early.

2) Request a PO amendment or new PO before doing extra work

The safest approach is: “Happy to proceed—please issue a revised PO reflecting the additional scope.” Many clients have a formal “change order” process. Use it. It protects both sides.

3) Document approvals in writing

Even if the client can’t update the PO immediately, get written approval for additional work and confirm they will update the PO before invoicing. This reduces disputes and gives your contact leverage internally.

4) Keep your invoice aligned with the updated PO

Once you receive the amendment/new PO, invoice against it with the correct PO number (or updated PO version). If there are multiple POs for one project, be very clear which invoice relates to which PO.

invoice24 is helpful here because it keeps your invoicing process organized, making it easy to create separate invoices for separate POs, include clear notes, and maintain a tidy paper trail—without needing complicated accounting software.

What to Do If the Client Requires a Vendor Setup

Some PO-driven clients won’t pay a new supplier until they’ve completed a vendor onboarding process. This can include collecting your business details, tax information, bank details, and sometimes compliance documents. It’s normal, especially for enterprise clients.

Common vendor setup requirements include:

Company registration details: Legal business name and address.

Tax information: VAT ID, EIN, or other identifiers depending on location.

Banking details: For bank transfer payments.

Insurance certificates: Sometimes required for contractors.

Signed supplier forms: Payment terms and policies acknowledgements.

To avoid delays, ask about vendor onboarding as soon as the client mentions a PO. If you wait until after the invoice is sent, it can add unnecessary time.

Once you’re onboarded, invoice24 helps you stay consistent by keeping your business details stored properly on every invoice, reducing the risk of administrative back-and-forth.

How to Send PO Invoices the Right Way

How you deliver the invoice can be just as important as what’s on it. PO clients often have strict submission rules because invoices are routed into automated systems. If you submit incorrectly, your invoice may not even enter the queue for matching.

Here are common invoice submission methods and how to handle each:

Email submission

Many companies use a dedicated email address like accounts@company.com or invoices@company.com. In these cases:

Attach the invoice as a PDF: Avoid sending invoices as images or in the email body alone.

Use a clear subject line: Include invoice number and PO number if possible.

Include minimal text: Keep the email short and factual to reduce confusion.

Supplier portal submission

Some clients require you to upload invoices to a portal. Portals often have strict field validation. Ensure the PO number is entered exactly and line items match.

Through your client contact

Sometimes a project manager wants you to send invoices to them for approval before forwarding to accounts payable. If that’s the process, still include the PO number and ask them to confirm the submission destination.

With invoice24, your invoices are generated cleanly and consistently, making them easy to attach, upload, and process. The less friction your invoice creates, the faster it moves.

Payment Terms and Timing with Purchase Orders

Purchase orders often come with standardized payment terms. This is important because your “payment clock” may start based on the invoice date, the invoice approval date, or the date the invoice is entered into the client’s system—depending on the client’s policy.

To protect yourself:

Read the PO payment terms: If it says Net 30, clarify whether that’s from invoice date or from acceptance date.

Invoice promptly: Don’t wait weeks after delivery. Many clients pay in scheduled cycles.

Follow up early and politely: Especially if you’re working with Net 45 or Net 60 and cash flow matters.

Ask about payment runs: Some companies pay only twice per month. Knowing this helps you plan.

invoice24 helps by keeping your invoice history organized, so you always know what you sent, when you sent it, and what’s outstanding—without needing advanced accounting tools.

Common Reasons PO Invoices Get Rejected (and How to Prevent Them)

If you want faster payment, you want fewer rejections. Here are the most common reasons PO invoices get rejected and what to do instead:

Missing PO number

Fix: