[bartwe]

Ensuring the code isn't reused, to put in a ledger, to avoid some kind of replay attack, having someone pretend to be the courier and get the key to then replay to the real courier. Dunno, probably many many things avoided by making the key not usable but still verifiable.

[trickstra]

I can think of only one scenario when it would make sense to destroy the bill - if the Drop is still able to present the whole bill after the Courier says they delivered the package, that is a proof the Courier is lying or got played and didn't verify the code. Drop is the one who has motivation to keep the bill intact, Courier uses the bill as proof of delivery, and cuts it in half just to be safe that someone from the Drop won't stop them at gunpoint after the delivery, stealing the whole bill.

[qrbLPHiKpiux]

Very important to never reuse a code, or one time pad. Perfect forward secrecy.