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.