The one that earns its keep: price
A customer sends a purchase order carrying last season’s price. Nobody notices, it ships, and the difference turns up at invoicing three weeks later — as an argument, a credit note, or a margin you quietly lost.
So you set how far a price on their document may sit from the price you agreed — as a percentage, a cash amount, or both. Inside it, the order goes through. Outside it, it stops, and the line says what the document claimed and what you actually charge.