Purchase orders
What is a purchase order?
A purchase order (PO) is the document a buyer issues before the work is done or the goods ship. It says what they are buying, how many, and at what price, and it carries a PO number. It is the buyer's approval to spend that money with you.
If you are the one sending the invoice, the part that matters is the PO number. You copy it onto your invoice, and the buyer's accounts payable (AP) team uses it to find the approved order and check your invoice against it before anyone pays. An invoice with no PO number, or the wrong one, usually waits in a queue or comes back to you.
So: the buyer creates the PO, you put its number on your invoice, and the amounts on the two should agree. If you do not have the number yet, ask for it before you send. JupiterInvoice has a PO Number field on every invoice, and your client can add the number themselves on the invoice you already sent.
What is a PO number?
A PO number is the reference number on a purchase order. The buyer's purchasing system creates it when the purchase is approved, and it identifies that one order. It can look like anything the buyer's system produces: plain digits, a prefix such as PO-, or a long code from accounting software. You do not make it up. You get it from the buyer.
What is a PO number on an invoice?
It is the buyer's purchase order number, copied onto your invoice so their AP team can find the order the invoice belongs to. Put it near the top, beside the invoice number and date, and label it "PO Number". Copy it exactly, with any prefix, dashes, or leading zeros, because AP software often matches it character for character.
What is the difference between a purchase order and an invoice?
The buyer issues the purchase order before the work, to say what they agree to buy. The seller issues the invoice after the work, to ask to be paid for what was delivered. A PO is a commitment to buy; an invoice is a request for payment. The invoice quotes the PO number, and its total should not be more than the PO allows.
What is a PO invoice?
A PO invoice is an ordinary invoice that quotes a purchase order number. It is not a separate kind of document. The phrase just means the buyer requires a PO number on the invoices they pay. Many larger companies have a "no PO, no pay" rule, so an invoice without the number is not processed.
How do purchase orders and invoices work together?
The buyer raises a PO and sends it to you. You do the work or ship the goods, and the buyer records that they received them. You send an invoice that quotes the PO number. AP then checks the three against each other: the PO, the receiving record, and your invoice. This is called three-way matching. If they agree, your invoice is approved and paid on your terms.
Do I need a PO number to invoice a client?
Only if your client uses them. Many small businesses and individuals never issue purchase orders, and an invoice without a PO number is a perfectly valid invoice. Larger companies often do require one, so ask before your first invoice whether they need a PO number. If they do and you leave it off, expect the invoice to come back unpaid.
Who issues the purchase order, the buyer or the seller?
The buyer. The purchase order comes from the company paying for the goods or services, usually from their purchasing or procurement team. The seller receives it, does the work, and then issues the invoice. If a client asks you to "send a PO", they almost always mean a quote, which they then turn into a purchase order on their side.
What happens if my invoice does not match the purchase order?
The invoice is usually put on hold or rejected, and AP goes back to you or to the person who ordered the work. The usual mismatches are a total higher than the PO, a different quantity or unit price, or the wrong PO number. If the work changed, ask the buyer to amend the PO before you invoice, or invoice up to what the PO covers and ask for a new PO for the rest.
Why does my invoice need a PO number to get paid?
Because the PO number is how the buyer proves the spend was approved. It ties your invoice to a budget and to the person who signed off on it. Without it, AP has no approved order to check your invoice against, so someone has to chase an approval internally before they can pay, and that is where invoices stall.
What is the best way to handle missing purchase order numbers on invoices?
Get the number before you send, not after a rejection. Ask for it when the work is agreed and again just before you invoice. If an invoice already went out without one, add the PO number to that same invoice rather than issuing a new invoice number, so the buyer does not see a duplicate. In JupiterInvoice the client can add the PO number on the invoice you sent, and you get an email when they do. More on that in adding a PO number after the invoice is sent.
Where the PO number goes on your invoice
- In the header block, next to the invoice number and invoice date, where AP looks first.
- Labelled "PO Number", not buried in the notes or the line-item descriptions.
- Copied exactly: same prefix, same dashes, same leading zeros. Check a number's format with the PO number checker.
- One PO per invoice. If the work spans two purchase orders, send two invoices.
- Line items described the way the PO describes them, so the lines can be matched one to one.
Why PO invoices get sent back
The common reasons, what AP sees on their side, and the fix that avoids a second round.
| Reason | What AP sees | The fix |
|---|---|---|
| No PO number | No approved order to match the invoice to | Ask for the number before you send, and add it to the same invoice if it arrives late |
| Wrong or mistyped PO number | A number that does not exist, or belongs to another supplier | Copy it from the PO document itself, not from an email thread |
| Total is higher than the PO | An invoice over the approved amount | Ask the buyer to amend the PO first, or invoice up to the PO and ask for a new one for the rest |
| PO already fully invoiced | A closed order with nothing left to pay against | Get a new PO for new work before you invoice it |
| Quantities or unit prices differ | Lines that do not match the PO lines | Mirror the PO's lines, quantities, and prices on the invoice |
| Billed to the wrong entity | An invoice addressed to a company that did not raise the PO | Bill the legal entity named on the PO, including its registered address |
| Goods or work not yet received | No receiving record, so the three-way match fails | Ask your contact to confirm receipt in their system, then AP can release it |
Make an invoice with the PO number on it
The builder has a PO Number field in the header, next to the invoice number. Fill it in, share the link or download the PDF. No account needed. If you do not have the number yet, send it anyway: your client can add the PO number on the invoice, and you get an email when they do.
Open the invoice builderThe long version
What is a purchase order?
A purchase order is a buyer's written authorization to buy. Here's how POs work, what goes on one, and how they pair with your invoice.
Last updated September 24, 2026
An invoice your client can put the PO number on
Every JupiterInvoice invoice has a PO Number field. If the number arrives late, the client adds it on the invoice you sent, and you get an email. Free, no signup.
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