Yes, it is good to keep your eye on the display as the items are scanned at checkout. I work for a large southeast supermarket chain that offers the item free if the scanned price is wrong. If you've purchased one, it would be free. If more than one, the first is free and the others are charged the correct price.
It's probably against the rules to mention the name of the store, since that's advertising, but I'll give you a clue: we offer carryout service to all of our customers (no tipping allowed).
