Both credit cards customer service departments assured me that I am not responsible for the charges that are not mine. Both have started investigating the frauds. I have to sign some sort of paperwork for the Mastercard fraud, that was sent out on Thursday, and Amex has already credited the Staples charge, so I don't have to pay for that either. According to both Customer Service departments, I'm not liable for any fraudulant charges, not even the first $50. Evidently the merchants are responsible.