There's also the concern of just how securely these sites treat your credit card data. I know of one that ignores your "forget my card after this transaction"

There's also the concern of just how securely these sites treat your credit card data. I know of one that ignores your "forget my card after this transaction" preference, and has you type your password in normal text field, and may possibly have led to fraudulent charges. Maybe this experience isn't typical but cough the user in question was hesitant to pay on other sites after that, even when the price would be totally reasonable.