You walk in the store and there's a whole basket of Elmer's Glue. The are regularly priced at $0.99 each and with the current sale, you get $0.99 instant savings, which means it's free.
I get two of them, walk to the cashier, hand them the glues (thinking... FREE? really?). They scan it, and the receipt prints out. "Thank you for shopping at Staples. Have a good day!".