JuneThree schools ask for a quotation on roughly the list they took last year. Somebody rebuilds each one in a spreadsheet, from the last one they can find.
JulyTwo are accepted. The stock is ordered from a total, not from a list, so the exercise books are right and the maths sets are short.
Late AugustThe counter is three deep with parents while the van is loading a campus delivery. The same reams are being sold twice out of one figure that nobody is updating.
SeptemberThe school queries eleven lines on the invoice. Nobody can put a hand on the quotation that was actually agreed, so the credit note is issued to keep the peace.
NovemberThe account is still unpaid, the margin on the contract is unknown, and the surplus maths sets are sitting where they will sit until next August.
What it actually costsThree hundred lines, one of them wrong, times forty orders
No single mispriced line on a school order is worth chasing. That is exactly why it survives — and why the contract margin is never quite what the quotation implied.