Can my customers choose their seats and add baggage when they book a flight on my portal?

Yes, in most cases they can, though how much choice they get depends on the airline. When a flight is searched on your portal, the airline or the supplier behind it sends back not just the fare but also the extras it is willing to sell at that moment, and seats and extra baggage are usually part of that. Your customer picks them during the booking steps, before payment, and the cost is added to what they pay you.

The honest limit is that not every airline shares the same amount of detail. Low cost carriers tend to be very good at this, because selling extras is a large part of how they make money, so you will often see a full seat map with prices per seat and several baggage options. Full service airlines vary. Some will let you request a seat but only confirm it later, some only release seat maps close to departure, and some will only accept a baggage request rather than sell you the bag outright. On a few fares, extras cannot be added at the time of booking at all, and the passenger has to do it with the airline directly or at the airport.

It also depends on which sources are switched on for your account. A booking flow that pulls from several suppliers may show a seat map for one airline and nothing for another on the very same search results page, which is normal and not a fault in your portal. Where an extra cannot be sold, the cleanest approach is to say so on the booking page so the traveller knows to arrange it themselves.

If this matters to the routes you sell most, ask which of your connected sources return seat maps and baggage options, and run a test booking on a route you know well to see exactly what your customer will be shown.