Can my customer pick their own seat on the plane when they book through your flight booking API?

Yes, in most cases, although it depends on the airline and on the fare your customer has chosen. The flight booking API can return a seat map for a selected flight, showing which seats are free, which are already taken, and whether a seat carries an extra charge. You can display that map on your own booking page so your customer taps the seat they want, and the choice is sent to the airline along with the booking.

How much you get varies quite a lot from one airline to another. Low cost carriers usually give a full seat map and charge for almost every seat, including the ordinary ones at the back. Full service airlines often release standard seats for free but hold back extra legroom and front rows for a fee or for their own loyalty members. A few airlines do not open seat selection to agents at all and only allow it at online check in, and on codeshare flights the airline actually operating the aircraft may handle seats differently from the one selling the ticket.

It is also worth being honest with your customer that a seat request is never a cast iron guarantee. If the airline swaps the aircraft, which happens more often than people expect, the seating layout changes and passengers get moved around. Paid seats are normally refunded or reassigned by the airline in that situation, but that is the airline's process, not something the booking stops from happening.

Before you promise seat selection on your site, check which airlines you sell most on your main routes and test a seat map request for each one so you know what your customers will actually see, and remind them to confirm their seat again at check in.