This is entirely up to you, but paying on the full amount is probably appropriate. Consider restaurant etiquette. If you have dinner at a nice restaurant and use a gift certificate, it is customary to tip the server based on the cost of your meal before you apply the gift certificate. They are still providing the same service to you, despite your reduced cost. Think of your Shipt shopper the same way. If you are using a $1 coupon, it obviously won’t make a huge difference, but if you are buying more expensive items that are on sale for “buy one get one free,” then you may want to think about padding the tip a little.