Purchase Order vs Invoice

The two documents side by side

Both list the same goods or services at the same prices. What differs is who writes them, when, and what they commit the writer to.

How the sequence normally runs

In an organisation that uses purchase orders, the flow is predictable, and each step exists to make the next one auditable.

• The buyer requests a quotation from you
• You send a quotation with scope, pricing and validity
• The buyer raises an internal purchase requisition and gets it approved
• The buyer issues a purchase order quoting a PO number
• You accept the purchase order and do the work
• You invoice, quoting that same PO number
• The buyer matches the invoice against the PO and the delivery record
• Payment is released on their next payment run

Why the PO number matters so much

Larger organisations run what is called three-way matching: the purchase order, the goods received note and the invoice must all agree before payment is released. It is an anti-fraud control, and it is automated.

An invoice arriving without a PO number typically cannot be matched, so it is rejected or parked by the system before a human ever considers it. Nobody is being difficult. The invoice simply has nowhere to go.

The practical consequence is that a missing PO number is one of the most common causes of a genuinely unexplained late payment. You chase, your contact says it was approved weeks ago, and the invoice has been sitting in an exceptions queue the whole time.

What to do before you start work

Ask two questions during the quoting stage rather than after delivery. First: do you require a purchase order for this? Second: which address should the invoice go to?

If the answer to the first is yes, do not begin work until you have the PO number in writing. Starting without one means you may have no authorised commitment to invoice against, and getting a retrospective PO raised can take weeks.

If the PO has a value limit and the scope grows, get the PO amended before you exceed it. Invoicing above the authorised amount will fail matching just as surely as invoicing with no PO at all.

Putting the PO number on your invoice

Put it somewhere prominent and clearly labelled, near your own invoice number rather than buried in a description line. "PO Number: 4500123456" is what the processing system and the person keying it in are both looking for.

Match the other details too. If the PO is addressed to a specific legal entity, invoice that entity, not the trading name you usually use. Mismatched entity names are the second most common matching failure after missing PO numbers.

In QuillBill you can add the PO reference in the document notes or as a dedicated line so it appears clearly on every template.

Do small businesses need to issue purchase orders?

Most sole traders and small businesses do not need a formal PO system for their own buying. A quotation you accept in writing serves the same purpose at that scale.

They become genuinely useful once more than one person can commit company money, or once you need to track committed spend against a budget before the invoices arrive. Below that, the administrative overhead outweighs the control.