If you use International Reply Coupons -- better known as I-R-C's – when defraygb the cost o return QSL cards -- then listen up. According to United States Pstal Bulletin 22184 dated July 6th 2006 not all the I-R-C's in your possession may be usable.
According to that bulletin, effective July 1, 2006, the Postal Service will no longer redeem international reply coupons issued prior to 2002. The non-redeemable IRCs are approximately 2.5 inches by 4 inches and do not have a barcode on the back.

The bulliten then goes on to request that retail associates advise customers possessing pre 2002 IRCs to return them to their correspondents in the country of issue. This, for redemption through the selling post office.
The Postal Service describes the current redeemable IRCs as being approximately 3.75 inches by 5 inches and can be distinguished by the presence of a barcode on the back. (VHF Reflrctor)