AMEX may charge the company. The merchant agreements are not always business-friendly. And yes, the stores may blacklist you for this as well. Mr. Porter has al in in their policies about banning people who return excessively.
Pho Bac (which was in the office building kind of place) closed a few years ago & cycled through a sushi restaurant & will soon be a burrito place. Pho King IV is closer to campus, but in a 1-story building, no offices.
I generally find that they process the return within a week of receiving it. If you use their in-house return label, it can take a while to get to them. If you have Shoprunner, it goes much more smoothly.