Can my customer choose their seat on the plane while booking a flight on my portal?

In many cases yes, but it depends on the airline behind the fare. Some airlines send their seat map through the flight API, so your customer can open a picture of the cabin during booking and pick a window or an aisle seat before paying. Other airlines do not share the seat map at that stage, and the seat can only be chosen later on the airline's own website using the booking reference, or at check in.

Where seat selection is available on the portal, free seats and paid seats are usually shown separately. If your customer picks a paid seat, the extra charge is added to what they pay you, and it is passed on to the airline with the booking. Low cost airlines tend to sell almost every seat this way, while full service airlines often keep a set of seats free for the passenger to take without paying.

There are honest limits worth knowing. Not every supplier in your portal will support seat selection, so the option may appear for one airline and not for another on the same search page. Seats for infants, for large groups and for some discounted fares are often blocked. Even after a seat is confirmed, an airline can move a passenger if it changes the aircraft, so a seat is a request the airline holds rather than a promise nobody can break.

Before you tell your customers they can pick seats, ask which of your connected airlines and suppliers actually return seat maps on your plan, and make one test booking with each so you know what your staff will see.