I mean you can say whatever you want. I usually go with baggage fees. It's vague but understandable enough.

IME the offshored customer service agents that handle this if you do it via message on the website don't really care. It probably will pass muster more if its a round number but I've never had a charge denied. I recently passed off a partial payment on a ticket and a couple of basic economy to regular economy co-pays as various baggage and seat assignment fees (not on BA, just as an example).