Credit password Voting: +1
Works even if you've lost your cash or credit card.
Credit cards contain no information that can't be memorized. Instead of cards, we could have simply long numeric passwords, which we entered upon transaction conveniently by securely touch-typing into a hidden numeric keypad. If the buyer's name in the credit provider's database matches the password, transaction is granted. Low tech, and more secure than a credit card:
To control the risk, the number of digits that has to be entered could depend on the size of the transaction. (To adjust for risk.) For example, buying a few candies would require 5 digits, whereas buying a car may require 9 digits, and the full-length password would be used rarely, if ever at all.