Can I pick my customer's seat and add a special meal when I book a flight through your API?

Yes, in most cases you can do both at the time of booking, but how much you are able to set depends on the airline you are booking.

For seats, many airlines send back a seat map through the flight booking API, so you can see which seats are open, which are free and which carry a charge, and pick one for your passenger while you are making the booking. Any seat charge is added to what you collect from the customer. Other airlines do not release their seat map to outside systems at all, or only open seats once online check in starts. When that happens the seat step simply will not appear, and the best you can do is give your customer the airline reference so they can choose a seat themselves on the airline website nearer to departure.

Special meals work a little differently. A request for a vegetarian meal, a child meal, a no beef meal or something similar is sent to the airline as a request, not as a confirmed item. Most airlines accept it when it is added early, but none of them guarantee it, and you should tell your customer that plainly so there is no argument later. Many short flights and many low cost airlines do not serve a meal at all, or sell food separately on board, so the option will not show for those.

Before you promise anything to a traveller, check which of the airlines you sell most often support seat selection and meal requests in your own setup, and get into the habit of adding these at the booking stage rather than days later, because the later you ask, the less likely the airline is to honour it.