I believe a merchant is required to disclose all fees before charging your card, yes. If AA did not, that's grounds to dispute the charge with AA and then with your card issuer.

Wow: AA's handling of this fee is unusually poor. Charging the fee to use vouchers and change tickets issued before the fee was introduced. No waivers for transactions that can't be done self-serve. And total failure to disclose the fee in any event, at least in some cases.