lowest
The default award rule: lowest bid, ties broken by vehicle name.
The tiebreak is the part that matters. Exact ties are not a curiosity in this subsystem -- a symmetric layout with identical vehicles produces them constantly -- and a rule that left them to the order proposals happened to arrive in would make the winner depend on the order the fleet was declared, which is not something a negotiation should be able to see. Name is intrinsic to the vehicle, stable under any reordering, and total.