Whenever I'm presented with stuff like this (another similar situation is being asked to apply for a loyalty card at most any store), I think of it this way - t

Whenever I'm presented with stuff like this (another similar situation is being asked to apply for a loyalty card at most any store), I think of it this way - the reason you have to ask me to do something is because it's a task that you want to make sure doesn't stand in the way of the rest of the transaction, meaning it's probably going to be something that's going to irritate me or something I won't want to do. Offering to bribe me a trifling sum in return means it's probably a serious pain in the ass, so of course I'm not going to do it. When someone has their hard-earned money out and is about to pay, they are cautious. They have made sure that they have gathered exactly what it is they want to purchase. They know that virtually anything out of the cashier's mouth or anything appearing on the checkout page that isn't "thanks, come again!" is likely to be something that's going to take time to do, cost them money, cause them to lose some of their privacy, receive junk mail, spam their friends, etc. He'd probably be better off simply putting a message on the "transaction completed" screen simply asking them if they could do whatever it is he wants them to do.