My rule #1 is to scan / photocopy ANY rebate information.
I would recommend a politely worded reply, or calling any number they listed in the reply or on the rebate form inquiring further about the "discrepancy." Sometimes the fine print can be misleading as to what is valid for the rebate.
Sometimes UPC codes get changed upon repackaging. This happened to me for a memory stick once. I called their 1-800 number, explained where and how I got it, and what other validation would they like. We did find out that in fact my UPC WAS valid, and they would update their information. (If they did who knows).
I had issues 3 other times and was successful in getting the rebate in 2 of the cases. I was in clearly error for the one I did not get (I knew it but tried anyway). I was also in error (for the fine print, my fault) for another, but got that rebate, after complaining about the way the worded the fine print, and being THAT picky. Their fine print was a "cheap shot" at getting out of the rebate, but I let them know that, and that I was dissapointed that they would do something like this and their failure to offer me and others the rebate based on the "cheap shot" would influence future purchases, and would be sure to let anyone know about their practices.
It was a little work, but worth $40 bucks to me.