Can my customers pick their seats when they book a flight through the flight booking API?

Yes, in most cases your customers can pick a seat while they book, although what you are able to offer depends on the airline and on the fare they have chosen.

Seat selection normally works in two steps. Once a flight is selected, the API returns a seat map for each part of the journey, showing which seats are free, which are already taken and which carry an extra charge. Your customer chooses a seat, and that request is sent to the airline along with the booking. Seats near the front, by a window or beside an exit are often paid seats, so the seat map also carries the cost of each one, and that amount is added to what your customer pays before the booking is completed.

The honest limit is that not every airline shares a seat map, and some release seats only at check in rather than at the time of booking. Low cost carriers and full service carriers behave quite differently here, and the same airline may allow free seat selection on one fare and charge for it on another. When no seat map comes back, the booking still goes through as normal, and the sensible thing is to tell your customer they can choose a seat later with the airline directly.

Before you promise seat selection on your website, it is worth asking us which of the airlines you are connected to return seat maps, and whether paid seats are included in your plan.