Solution: Proof of purchase for some contests - NYT Crossword

I was surprised to learn that this was a debut; advertising ephemera is a whole collectible category and companies have been offering prizes and gifts in exchange for a BOXTOP, as a proof of purchase and loyalty, for a long time. In 1851, a soap manufacturer publicized his product by wrapping bars of soap for the first time — they had been nakedly stacked on shelves or piled in barrels. If a buyer sent in enough wrappers they would get a floral lithograph. This proved to be such a gangbusters concept that the whole concept of cereal boxes and other packaging took inspiration, and boxtops could be traded for books, sports gear and even land in the Yukon.