Basics

Purchase orders: why the PO number must be on your invoice

What a purchase order is, why larger customers will not pay an invoice without its number, and how to keep track of what is left on one.

7 Oct 2026 · 6 min read

One purchase order for $12,000. Each invoice that carries its number uses up part of it.

You finish the work, send the invoice and wait. Thirty days later nothing has arrived. You ask, and the answer is one line: “We cannot pay this, there is no PO number on it.” The work was fine and the amount was right. The invoice simply could not get through the customer's accounts team. This happens to almost everyone the first time they work for a larger company.

What a purchase order is

A purchase order, or PO, is a document the buyer sends the seller before the work starts. It says what they are buying, how much they will pay and on what terms. It has a number. Inside the buyer's company that number is the approval: it shows that someone with the authority to spend the money has agreed to spend it.

A quotation goes from you to the customer and says what you will charge. A purchase order comes back from the customer and says they accept. An invoice goes from you to the customer afterwards and asks for the money. If you are unsure about the first of these, see quotation, proforma and invoice.

Why they will not pay without the number

In a small business the person who ordered the work is often the person who pays for it. In a larger one they are different people. The accounts team that pays you has never heard of your project. All they can do is check three things against each other: the purchase order, proof that the work or goods arrived, and your invoice. The PO number is what ties the three together.

An invoice with no PO number matches nothing. It is not refused out of bad will. It waits in a queue until someone works out who ordered it, and that can take weeks.

The PO number sits near the top of the invoice, under its own label, exactly as the customer wrote it.

Where the number goes

Near the top of the invoice, under a label that says PO number, not buried in the notes.
Written exactly as the customer wrote it, with the same letters, dashes and zeros.
One purchase order for each invoice. If the work falls under two, send two invoices.
In the subject of the email as well, if you send the invoice by email.

The rest of what an invoice needs is in our checklist of what every invoice must show.

Ask before you start

The easiest time to get a PO number is before the work begins. Ask one question when the customer says yes: “Do you need a purchase order number on my invoices?” If they do, ask for the purchase order itself, not only the number, and read it. Check four things:

The amount. It should cover the whole job, including tax if they count tax inside it.
The dates. Many purchase orders end on a date. Work invoiced after it may not be paid.
The payment terms. If the PO says 60 days and your quotation said 30, settle it now. See invoice payment terms.
Where to send invoices. Often a separate accounts address, not your contact's own email.

Keep track of what is left

A purchase order is a budget. Each invoice you send against it uses some of it up. When it runs out, the next invoice is stopped, even if the work was asked for. So keep a running total, as in the picture at the top: the value of the purchase order, each invoice against it and what is left.

When you can see the end coming, tell the customer early. “We have $2,000 left on PO-48213 and about three weeks of work to go. Could you raise a new purchase order or extend this one?” A request like that is easy to approve in advance and very hard to sort out afterwards.

When the work goes past the purchase order

Extra work was agreed. Ask for the purchase order to be increased, or for a new one, before you invoice the extra.
You already invoiced more than was left. Ask which they prefer: a new PO for the whole invoice, or the invoice split in two.
An invoice went out with the wrong number. An issued invoice should not be edited. Cancel it or credit it and issue it again with the right number. See how to fix a wrong invoice.

In Openn InvoiceOn a new invoice, open PO number, ship-to & notes and type the number. It prints on the invoice. To keep track of a purchase order's value, open Projects, then the Purchase orders tab, and choose Add purchase order. Every invoice with that PO number counts against it, so you can see what is left.

The short version

A purchase order is the buyer's written approval to spend, and it has a number.
Larger customers match your invoice to it. With no number, the invoice waits.
Ask for the purchase order before you start, and read the amount, dates and terms.
Put the number near the top of every invoice, exactly as written.
Keep a running total, and ask for more before it runs out.