Can I choose my customer's seat on the plane through the flight booking API, or do they have to do that with the airline?

Yes, with many airlines you can, but not with all of them. When an airline shares its seat map, the flight booking API shows you which seats are free, which ones have extra legroom and which ones carry a charge, and you can pick a seat while you are making the booking. When an airline does not open seat selection to agents, no seat map comes back and your customer has to choose a seat on the airline website or at check in.

How it works also depends on the fare. On low cost airlines a seat is almost always a paid extra, and the API returns the cost of each seat so you can add it to the booking the same way you add baggage. On full service airlines a standard seat is often free once the ticket is issued, while exit row and front row seats are sold separately, and on the cheapest fares some airlines hold seat selection back until check in.

It is safer to treat a seat as a request until the airline confirms it. If the airline changes the aircraft or reschedules the flight, the seat can move or be lost, and the API will show the new seat only after the airline updates the booking. Seats on flights that are operated by a partner airline are often not selectable through the API at all.

Before you promise a seat, check which of the airlines you sell most often return a seat map in your account, and tell your customer that the seat is final only when it shows on their ticket or boarding pass.