The short answer
A purchase order is the client's internal authorisation to spend money with you. If their finance system expects one and your invoice does not carry the number, the invoice does not get rejected so much as it never enters the queue. Nobody tells you, because from their side nothing has arrived.
Ask during the project setup whether they raise purchase orders, and get the number before you invoice.
Put the number on the invoice itself, in a field labeled as they label it, not in the email body.
Match your invoice to the wording and the amounts on the purchase order line by line.
If you already invoiced without one, ask your contact to raise it, then reissue the invoice against it.
Expect a supplier portal alongside it. Registration is often a separate blocker with its own delay.
This is one of the quietest reasons an invoice stalls, and one of the easiest to prevent. It costs a single question at the start of a project.
What a purchase order actually is
A purchase order is raised by your client, not by you. Somebody inside the company decides to buy something, a budget holder approves it, and their system issues a numbered document that commits the money in advance. The number is the handle everything else hangs from.
The reason finance teams care so much is a control called the 3-way match. Before an invoice is paid, accounts payable lines up 3 documents and checks they agree:
The purchase order, which says what was authorised and at what price.
The receipt or acceptance record, which says the work or goods actually arrived.
Your invoice, which asks to be paid for it.
If the 3 agree, the invoice pays on schedule with no human judgment involved. If any one of them is missing, the match fails and the invoice sits in an exceptions queue waiting for somebody to intervene. An invoice with no purchase order number fails the match at the first step.
This matters for services as much as for goods. Agencies often assume purchase orders are a manufacturing thing, then discover that their client's procurement system treats a brand campaign exactly like a pallet of parts.
How to tell whether your client uses them
Ask. It is a routine operational question and no client will think worse of you for it. The right moment is during setup, alongside the other payment questions worth asking before you start:
Do you raise purchase orders for supplier work like this?
Who raises it, and who approves it?
Will the purchase order cover the whole engagement or each phase separately?
What should the invoice reference, and where should it go?
There are signals even before you ask. A company with a procurement department, a supplier code of conduct, a signed master services agreement or anything resembling a vendor portal is almost certainly running purchase orders. So is most of the public sector, most large enterprises, and most companies owned by a larger group.
The awkward middle is the mid-sized client whose contact is a marketing manager who has never raised one. They will tell you in good faith that no purchase order is needed, and they will be wrong, because the requirement lives in a finance policy they have never read. Above a few thousand dollars, ask them to check with finance rather than taking the answer at face value.
Getting the number onto the invoice properly
Having a purchase order number and having it in the right place are different things. Accounts payable systems increasingly read invoices automatically, and an automated reader looks at fields, not at prose.
Put it on the invoice document, near the invoice number, labeled the way the client labels it. If they say PO, write PO. If they say order reference, write order reference.
Do not bury it in the email. The email body is frequently discarded when the attachment is filed.
Match the amounts. If the purchase order is for $12,000 and you invoice $12,400 for an extra round of work, the match fails on the difference. Get the purchase order amended first.
Match the description. Invoice against the wording on the purchase order lines, even where your own naming would be clearer.
Watch the expiry. Purchase orders can be issued for a period or a budget year. A purchase order raised in one year may not be spendable in the next.
The last 2 points are where long engagements fail. A retainer billed monthly against a single annual purchase order works until the scope grows, and then every invoice above the remaining balance quietly stops matching.
What to do when you have already invoiced without one
This is the common case, and it is recoverable. The invoice is not refused, it is unprocessable, which means the fix is administrative rather than a negotiation.
Go to your contact rather than to accounts payable, because your contact is the only person who can get the purchase order raised:
Subject: Invoice #214 — purchase order reference
Hi [Name],
Invoice #214 for [amount] was sent on [date] and I don't think it can be processed, because it doesn't carry a purchase order number.
Could you confirm whether a purchase order is needed for this work? If so, would you be able to raise one for [amount] covering [description]? Once I have the number I'll reissue the invoice against it the same day.
The invoice date stays as it is, so the payment terms shouldn't move.
Thanks,
[Name]
2 details make this work. It asks the client to do something small and specific rather than to explain themselves, and it raises the question of the due date before they do. Some clients will reset the clock to the reissue date. Raise it early and you have a chance of keeping the original date; raise it after 60 days and you will not.
If your contact goes quiet on this, it is usually because raising the purchase order means admitting internally that work was commissioned without one. That is their problem to manage, but it is worth knowing it is the reason, because it changes the tone you use. The approach in moving an invoice from a vague assurance to a named owner applies here too.
The other blocker: supplier setup
A purchase order is half the story. Larger clients also require you to exist in their system as an approved supplier before anything can be paid to you, and the 2 processes run separately.
Supplier onboarding usually means submitting your company details, bank details, tax registration and sometimes insurance certificates, either on a form or through a procurement platform. Coupa and SAP Ariba are the platforms agencies meet most often, and Coupa's supplier portal is free to suppliers, letting you see purchase orders and submit invoices against them directly.
Onboarding is slow in a way that surprises people. Bank detail verification alone can take weeks, because it is the step most exposed to fraud and is deliberately made awkward. Start it the day the work is agreed, not the day you want to invoice.
Where a portal is in use, sending a PDF by email is often the same as sending nothing. Ask which channel is the real one, and send it there.
What makes an invoice payable at all
Purchase orders are part of a wider idea: finance teams pay against a defined standard, and an invoice that misses it is set aside rather than argued with. The clearest published version of that standard is the one the US government sets for its own suppliers.
Under the Federal Acquisition Regulation, a proper invoice has to carry the contractor's name and address, the invoice date and number, the contract number including order and line item numbers, a description with quantity, unit of measure, unit price and extended price, the payee's name and address, and a contact for questions. Payment is then due on the 30th day after receipt of a proper invoice or after acceptance of the work, whichever is later. If an invoice is not proper, the billing office has to return it, with reasons, within 7 days.
That last rule is worth noting, because private-sector clients are under no such obligation. A corporate accounts payable team can hold a defective invoice indefinitely without telling you, which is why silence so often means a formatting problem.
In the EU, Directive 2014/55/EU requires public authorities to receive and process electronic invoices that meet the European standard, EN 16931. In the UK, the Procurement Act 2023 implies a 30-day payment term into public contracts, and the government's guidance is explicit that the 30 days run from the day the invoice is received rather than the day it is approved. The same guidance notes the clock does not apply where the authority considers the invoice invalid or disputes it, which brings you back to getting the details right at the point of sending.
Where the purchase order sits in the sequence
Set against the other things that hold an invoice up, the purchase order problem is distinctive in being entirely preventable and entirely invisible:
Blocker | When you find out | The fix |
|---|---|---|
No purchase order raised | Weeks after invoicing, if you ask | Ask at setup, get the number before invoicing |
Purchase order number missing from the invoice | When the match fails silently | Put it in a labeled field on the document |
Invoice exceeds the purchase order amount | When the balance runs out mid-engagement | Amend the purchase order before the extra work |
Not set up as a supplier | When accounts payable has no record of you | Start onboarding the day work is agreed |
Sent to the wrong channel | Never, unless you ask | Confirm the portal or the billing address |
Each of these is a version of the same thing: the invoice is not in the system that pays invoices. The other 6 reasons invoices stall are mostly about approval and timing once it is in there.
Building it into how you work
Treat the purchase order as part of project setup rather than part of billing. Ask the question during onboarding, record the number with the project, check it against the value before each invoice, and flag it when the remaining balance gets thin. Sending invoices from a shared billing address helps, because purchase order numbers and portal credentials then live somewhere other than one person's inbox, and agreeing payment terms that state when the clock starts settles the reissue question before it comes up.
FAQ
What is a PO number on an invoice?
It is the reference of the purchase order your client raised to authorise the spend. Accounts payable uses it to match your invoice to the approved order and to the record that the work was delivered. Without it the match fails and the invoice waits for manual handling.
My client says they need a PO number and I don't have one. What do I do?
Ask your contact to raise the purchase order, then reissue the invoice quoting it. Do this quickly and ask whether the original invoice date still governs the payment terms, because some clients restart the clock from the reissued invoice.
Do small agencies need to use purchase orders?
You never raise one yourself, so the question is only whether your client uses them. Any client with a procurement function, a vendor portal or public ownership probably does. For a 5-person client, probably not.
Can I invoice for more than the purchase order says?
Not successfully. An invoice above the purchase order value fails the match and stops. Ask for the purchase order to be amended or for an additional one to be raised before you deliver the extra work.
How long does supplier onboarding usually take?
Longer than the work often allows, because bank detail verification is deliberately slow. Treat it as something to start when the engagement is agreed rather than when the first invoice is ready.
Why did nobody tell me my invoice was missing a PO number?
Because most private-sector clients have no obligation to. US federal agencies must return a defective invoice with reasons within 7 days, but a corporate accounts payable team can leave it unprocessed without notice.
Not legal or tax advice. Procurement rules and payment legislation vary by country and by contract — check the terms you have actually signed, and take advice on anything formal.