Honestly if it clearly stated that the certificate can only be used on full retail purchases then it sounds like the OP is a moron for bitching about this. It would be different if it did not state any conditions on the certificate. How big of a deal is it to buy your damn parts that are already being price matched and then toss in a U-joint or two for future use to use your certificate on.
Certain situations you really need to be a business owner to understand. Customer service is important and i agree that if they are going to donate these certificates they really should not have restrictions like this but at the end of the day, in this situation at least, the OP is at fault for not being capable of reading.