How to Invoice Against a Purchase Order

Last updated 26 August 2026 · By InvoiceSonic Team · 7 minute read

Quick answer

Use the supplier and buyer details from the approved purchase order, place the PO number prominently on the invoice, and match descriptions, quantities, prices, currency and tax treatment to the PO. Invoice only what has been supplied or the milestone that has been accepted, then send it to the billing channel named on the order.

Purchase order versus invoice

The buyer issues a purchase order to authorise a purchase; the supplier issues an invoice to request payment. The documents should correspond, but they serve different purposes. Do not copy a PO number onto an invoice unless the buyer actually issued it for that work.

Create the PO-backed invoice step by step

  1. Confirm the PO is approved and belongs to the correct buyer entity.
  2. Copy the PO number exactly and display it near the invoice number.
  3. Match item descriptions, quantities, unit prices, currency and tax treatment.
  4. Invoice only delivered goods, accepted work or the authorised milestone.
  5. Add delivery-note, timesheet or project references requested by the buyer.
  6. Send the invoice through the portal or accounts address stated on the PO.
  7. Keep the PO, invoice and evidence of delivery together.

Partial deliveries and multiple invoices

When one PO covers several deliveries or milestones, show the quantity or value invoiced now and identify the relevant PO line. Do not imply that the entire order has been billed if a balance remains.

Use a new invoice number for each request. A note such as “PO 4500182, line 3, milestone 2 of 4” gives the buyer enough information to reconcile it.

Resolve mismatches before submitting

Accounts-payable systems commonly compare the PO, evidence of receipt and invoice. Differences in legal entity, quantity, rate, currency or tax can stop approval. Ask the buyer to amend the PO rather than altering the commercial facts on your invoice to force a match.

Need the buyer-side document first? Use the purchase order generator.

Frequently asked questions

Where should the PO number go on an invoice?

Put it in the invoice metadata near the invoice number and dates. If the PO has several lines, repeat the relevant PO line or project reference in the line-item description.

Can one purchase order have multiple invoices?

Yes, when the PO permits staged deliveries, recurring periods or milestones. Each invoice needs its own number and should state which portion of the PO it covers.

What if the invoice total is higher than the PO?

Pause and obtain an amended PO or written authorisation through the buyer’s process. An unexplained overage is likely to be rejected.

Can I invoice without a purchase order?

Only if the buyer does not require one or has authorised an exception. Ask before doing the work because some organisations cannot approve supplier invoices without a valid PO.

Related guides

Purchase order generator →How to send an invoice →Invoice payment terms →Progress billing →

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