Will I be able to see the cancellation charges for a flight before my customer books it?

Yes, in most cases you will see the cancellation and change charges for a fare before your customer pays, but how much detail you get depends on the airline and the supplier behind the search.

When your portal searches for flights, each result usually comes back with the fare rules attached, or with a small extra call that fetches the rules for that one fare. Those rules tell you whether the ticket is refundable, how much the airline keeps if the customer cancels, what it costs to change the travel date, and whether any part of the fare is lost whatever happens. Your portal can show that text on the review page, so your customer reads it before entering card details.

The honest limit is that airlines do not all write these rules the same way. Some send a clean set of conditions your portal can display neatly, and some send long blocks of text written for airline staff rather than for travellers. Low cost carriers in particular often give very short rules, and a few send nothing useful at all through the API. In those cases the safest thing to show is that the fare is non refundable, or that charges apply as per the airline policy, rather than something specific that might turn out to be wrong.

It is also worth remembering that the airline can apply its own handling charge on top, and that anything your agency charges for the cancellation is separate and should be shown separately so nobody is surprised later. Before you go live, run a few test searches on the routes you sell most and read what the fare rules actually say, so you know what your customers will see.