Payment terms are agreed once and stated in two places, and the two frequently disagree. Your purchase order says what you agreed; the supplier's invoice says what they would prefer. Which one governs is a commercial and legal question, and the practical point is that having your terms on the order is what makes the conversation short.
Why the two documents differ
Suppliers print their standard terms on every invoice regardless of what was negotiated, because their invoicing system does it automatically. That is not usually an attempt to change anything; it is a template. But an invoice stating shorter terms than your order will produce chasing calls timed to those terms, and somebody has to explain the difference every time.
What to put on the order
The payment period and from what event it runs, such as from receipt of a correct invoice or from month end. Any early settlement discount and its conditions. And the delivery expectation, since late delivery and payment terms interact. All of this fits in a short block and prevents a category of recurring disagreement.
When the terms are actually in dispute
Where a supplier's own conditions were accepted at contract stage, or where a framework agreement governs, the order's terms may not be the operative ones. That is a legal question about your particular arrangement rather than a payables one, and it is worth resolving with whoever advises you before an invoice is overdue rather than after.
Questions people ask about purchase order payment terms
What terms are standard?
They vary widely by sector and by jurisdiction, and some sectors have statutory rules. Establish what applies to you rather than assuming a norm.
Should we pay early for a discount?
Only where the discount exceeds your cost of capital and the cash is available. It is a finance decision.
What if a supplier insists on their terms?
That is a commercial negotiation, ideally settled at contract stage rather than invoice by invoice.