[Rick Lee]

Tipping on a credit card is fine. At the end of the shift all their checks get rung up and the servers pay out. The amount paid on credit cards is deducted out of what they owe the house. Servers don't take diners' cash and immediately put it in the register. They keep it as their "bank" and use it to give back change and pay the house at the end of the shift. After they've paid out, what's left is their net tips. If every single diner paid by credit card and no cash ever changed hands, the server would be paid out the difference between the total of the rung up checks and the final credit card receipts.