The path from a purchase order to a paid invoice is short when three documents exist and long when any of them does not. The order says what was agreed, the receipt says what arrived, the invoice claims payment, and a single reference number joins them. Everything that makes this process expensive is one of those four things being absent or wrong.
The path when everything exists
The invoice arrives quoting the order number, is captured and validated, is compared against the order and the receipt at line level within tolerance, is routed by rule to an approver or clears without one, is coded from the order and scheduled for payment. Minutes of machine work and no research.
The path when the reference is missing
Somebody must establish what the invoice is for, whether it was authorised, who owns the spend and which cost centre carries it. That is research involving at least one other person and usually a day or more of elapsed time. It is why organisations return unreferenced invoices rather than absorbing the work quietly.
The path when the receipt is missing
The match cannot complete, so the invoice waits while somebody establishes whether the delivery arrived. This is the commonest single cause of an aged exception, and the fix is upstream: recording receipts at the point of delivery rather than batching them into finance later.
Questions people ask about purchase order invoice process
Which absence is most expensive?
A missing order, because it turns processing into research. A missing receipt is more common but usually resolves with one phone call.
Should invoices without references be returned?
Many organisations do. Decide it as a policy, tell suppliers in advance, and be consistent, or it just produces friction.
Can the reference be added later?
Yes, as a correction with a note. Guessing which order an invoice belongs to is worse than asking.