When it comes to leaving tips when redeeming a coupon, voucher or even gift card, the total tip should always be calculated on the total amount consumed — before sales tax. Why not on just the amount you pay? After all, you didn’t PAY full price, so why tip full price? Right? No – WRONG!
You tip your waiter or waitress based upon service. The food they serve and the service the provide is not discounted or cut in half. They don’t take care of you half the time. They do not fill your drinks only half full. They provide a FULL service to you when you dine out. They are owed the full amount of their tip, regardless of what you spend.