trading stamp

American Heritage® Dictionary of the English Language, Fourth Edition

1. 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.

Wiktionary

1. n. a small paper coupon, given by a retailer to a customer; they may be saved up and redeemed for goods

WordNet 3.0

1. n. a token resembling a stamp given by a retailer to a buyer; the token is redeemable for articles on a special list