Can the flight booking API tell me what luggage is included in a fare and let me add extra bags?

Yes, in most cases the flight booking API returns the baggage allowance along with the fare, and on many airlines your customer can add extra bags at the time of booking.

When you search, each fare that comes back usually carries its own baggage details, such as how many cabin bags are allowed and whether checked baggage is included. Some airlines describe the allowance as a number of pieces and others describe it as a total weight, so the wording your portal shows will follow whatever the airline sends. It is worth showing that text to your customer exactly as it arrives, because a cheaper fare on the same flight is very often the one with no checked bag at all.

Adding extra baggage is where it depends on the airline and the supplier. Many low cost carriers sell extra bags through the connection, so your staff can add them while booking and they are confirmed with the ticket. Other airlines only sell extra baggage on their own website or at the airport, and in those cases the API will not offer it. Allowances can also differ between the outbound and the return, or between two airlines sharing one ticket, so it is safer to read the allowance for each leg than for the trip as a whole.

Before you go live, run a few test searches on the routes you sell most, check what baggage information actually comes back, and ask your supplier which airlines allow extra bags to be bought through the connection.