Your client, Zitzitz Inc., issues its own trading stamps along with the sale of merchandise. These stamps are redeemable at Zitzitz when a book is filled. Thus, with each sale Zitzitz creates a liability in the amount of the value of the stamps. However, not all stamps will ultimately be redeemed by customers. What should you, the auditor, do to satisfy yourself that the liability for redemption of stamps carried on the books of Zitzitz is adequate?