Can I show my customer how much baggage is included in the fare and let them add an extra bag while they book?

Yes, in most cases. The baggage allowance comes to your portal from the airline along with the fare, so your search results and your booking page can show whether cabin and checked bags are included before your customer pays anything. Because the allowance travels with the fare and not with the route, two seats on the very same flight can carry different baggage, and the cheapest option on the page is often the one that includes the least.

Selling an extra bag during the booking is a separate thing, and here the honest answer is that it depends on the airline. Some airlines allow extra baggage to be sold at the time of booking, and where that is the case your customer can choose it on your portal and pay for it together with the ticket. Others only sell it afterwards on their own website or at the airport counter. Your portal can only offer what the airline itself allows through the connection you are using.

Budget airlines behave differently again. With them a checked bag is normally a paid extra rather than something included in the fare, so it matters that your booking page says so clearly. If it does not, your customer will assume a bag is included and will come back to you unhappy after being asked to pay at the counter.

Before you go live, run a few test searches on the airlines you sell most often and read what your booking page tells the customer about bags, then ask your supplier which of those airlines accept baggage added at the time of booking.