trading stamp
n.
A stamp given by a retailer to a buyer for a purchase of a specified amount and intended to be redeemed in quantity for merchandise.

[TahlilGaran] American Dictionary ▲

trading stamp
n. One of the stamps that you get (as from a store or gas station) because you buy something there; a stamp you get with a purchase and save in special books until you have enough to take to a special store and trade for something you want.
Mother always buys things in stores where they give trading stamps.

[TahlilGaran] English Idioms Dictionary ▲