PO Number on an Invoice: Where It Goes and What to Check

A purchase-order number, or PO number, identifies the order issued by your customer. Add the customer’s PO reference to your invoice so their accounts-payable team can match your bill to the approved purchase. Keep your own invoice number as a separate reference.

Prepared September 4, 2026. The reference numbers and job examples below are fictional. Stripe’s guide to PO numbers explains how a customer-issued purchase order links to the supplier’s invoice.

How is a PO number different from an invoice number?

Your customer supplies the PO number; your business assigns the invoice number. A client may use one PO for several approved deliveries or service periods. In that case, each invoice still needs its own invoice number even when the PO reference stays the same.

Keep the PO reference exactly as supplied, including leading zeros or letter prefixes. Do not replace INV-1042 with PO-7831. Keeping both lets you identify the bill in your records and lets your customer find the corresponding order in theirs.

Where should the PO number go?

Use the invoice’s dedicated purchase-order field when available. Otherwise, label the reference clearly in a visible notes or reference area: “Customer PO: PO-7831.” Check the exported PDF to make sure it appears on the document your client will process. A reference mentioned only in an email subject may not travel with the invoice into the client’s billing system.

Before creating the bill, ask the client whether their purchasing process requires a particular field, supplier portal, or invoice format. A note on a PDF will not satisfy a mandatory portal field. Follow the customer’s submission instructions and retain a copy of what you submitted.

What should match before you send the invoice?

For a fictional office maintenance job, PO-7831 covers two approved visits at $225 each. Invoice INV-1042 should identify those visits, show the agreed rate, and total $450 before any applicable tax or other agreed charges. If you completed only one billable visit, do not invoice both simply because the PO authorizes two.

1. Confirm the customer entity and billing address against the order.
2. Copy the PO reference and any requested project or department code.
3. Match the work description to the order without hiding meaningful differences.
4. Check quantities, rates, currency, and any separately approved charges.
5. Confirm the work or milestone has reached the agreed billing point.
6. Attach or reference completion evidence if the customer requests it.
7. Send through the required channel to the correct billing contact.

If the client added a third visit, keep the approval for that change and ask whether the PO must be updated before billing. Do not quietly change the quantity to make the invoice total fit an old order.

What if the client has not supplied a PO?

Ask whether a PO is required and who can provide it. Some customers do not use purchase orders; others cannot process a bill without one. Do not invent a customer PO or reuse an unrelated reference just to fill a field.

You can adapt this request:

I’m preparing the invoice for the two completed maintenance visits. Does your billing team require a purchase-order number or supplier-portal submission? Please send the reference and submission instructions so the invoice reaches the right queue.

This message is a draft to use before submission. If work has already been billed and the missing PO caused a rejection, confirm the correction process with the billing team. Make it clear that the resubmitted invoice replaces the rejected copy rather than creating another charge.

Does adding a PO number guarantee approval or payment?

No. A reference helps the client find the order, but it does not confirm that the invoice matches the approved work or that payment has been scheduled. Ask which specific field or approval is missing if a bill is held. Keep the explanation tied to the invoice and PO references so the next person reviewing it can follow the record.

How can you prepare the invoice with WaffleInvoice?

Start with WaffleInvoice’s free invoice tool to lay out your client details and charges. Use a clearly labeled PO reference in the notes if that format meets your customer’s requirements, then preview the document before saving it as a PDF through your browser’s print dialog.

The free tool prepares an invoice; it does not submit bills to your customer’s procurement portal or perform automated PO matching. Prepare your invoice once you have the customer’s instructions.

Related: Write clear invoice line items · Summarize several invoices in a statement.