Can I see how much baggage is included in a fare and sell extra bags through a flight booking API?

Yes, in most cases you can see what baggage is included in a fare through a flight booking API, and with many airlines you can sell extra bags as well. When the search results come back, each fare usually carries its baggage allowance along with it, saying whether it is cabin baggage only or whether checked baggage is part of the fare too. Showing that on your own search page helps your customer choose the right fare instead of finding out at the airport counter.

How much detail you get depends entirely on the airline and on the supplier feeding you the fare. Some airlines return a clear allowance for every fare family, so you can tell your customer exactly what they are entitled to carry. Others send back very little, or send it only once the booking has moved further along, and in those cases the safest thing is to show what you have and ask the customer to confirm anything beyond it with the airline.

Selling extra baggage is a separate step from selling the seat. Where the airline supports it, the API offers paid baggage as an add on during the booking, and you collect the money for it the same way you collect the fare. Low cost carriers tend to support this well, because extras are a large part of how they earn. Full service airlines are more mixed, and some of them will only let the passenger add bags on the airline website after the ticket has been issued.

The sensible next step is to ask which of your airlines return baggage details in the search response and which of them allow paid bags to be added at the time of booking, then decide how you want to display that on your pages.