Invoice vs purchase order

The difference in one line

A purchase order is the buyer saying "send me this, at this price". An invoice is the seller saying "I sent it, now pay me".

The order of events

• The buyer raises a PO with a PO number, the items, the agreed prices and a delivery date.
• The seller accepts it and does the work or ships the goods.
• The seller invoices, quoting the PO number so the two documents can be matched.
• The buyer's accounts team checks invoice against PO against delivery, then pays.

Why the PO number matters so much

In any company large enough to run purchase orders, an invoice without a valid PO number sits unpaid. It isn't obstruction — the accounts system genuinely has nowhere to put it. If a client mentions a PO, get the number before you invoice, and put it on the invoice where it can't be missed.

Do small businesses need POs?

As a seller, only if your customers use them. As a buyer, they're useful whenever you're ordering materials on account: the PO fixes what you agreed, so a supplier invoice that doesn't match is easy to challenge.

Make either document

Raise one with the purchase order generator and bill with the invoice generator. Put the PO number in the notes field so it prints on the PDF.