USA Gift Certificates offered in trade

[J.Allen]

I have a small business that sells its products at small conventions. At some shows we offer gift certificates/coupons to the convention in exchange for all or part of our booth fee and the convention gives these certificates to their staff and volunteers as a thank you for helping out. When we do this, we usually give out a larger amount in certificates than we would pay if we paid cash for the booth, but there there are usually a portion of the certificates that are not redeemed by the end of the show. For example, our booth may cost $50 if we paid cash, but instead we give the event $75 in gift certificates. By the end of the show generally 80 - 90% of the certificates will have been turned in. Since these certificates are handed out on Friday and must be used by Sunday, we don't bother to record the liability. What is the proper way to account for the certificates that are redeemed?