Lock the best supplier rate that satisfies a sold offer, at the total the offer showed. The traveler chose an option, not a supplier: travel.tech asks the provider for a lock on one candidate rate at a time, checks the locked terms (board, cancellation, free-cancellation deadline, merchant, price) against what was sold, and stops at the first that satisfies them. A candidate that fails is passed over without the traveler seeing it. The response is the locked fare (StayFareOut) at the offer's total; book it with the fare id it returns.

409 PRICE_ACCEPTANCE_REQUIRED means a rate satisfies the offer but not at the price shown: details[0] carries previousTotal, newTotal and cause, and message says it in one line. Lock again with acceptedTotal set to newTotal and a new Idempotency-Key (reusing a key with another acceptedTotal answers 422). 410 OFFER_EXPIRED with recoveryAction REFRESH means no rate behind the offer can be locked; search again. 409 OFFER_LOCK_IN_PROGRESS means another call with the same key is still working.

Idempotency-Key is required. Replaying a key for the same offer returns the first answer with no provider call. The route answers 404 while the feature is off.