Well, from my understanding, due to the junk that is AC IT, you will be charged for extra bags as you didn't book the ticket with your points. Ideally you present your E50K card and they waive it for you. However, FT members have reported saying that they are forced to be charged, and then they try to go after AC to get a refund.

My tip? Use an AMEX, get charged, then dispute it. I would do this, and tell AMEX the truth. Tell them the whole situation so morally you can sleep at night. If you don't want to go abouts doing this, just dispute the charge and say you didn't authorize it. Obviously, one has a higher chance of success, but one is morally more unethical. Some will say my suggestion of doing a chargeback is also entirely morally wrong, but hey, AC [expletive] up in the first place