They're probably trying to bluff you thinking you didn't make a copy of the invoice. Since you have one, you'll be fine in the long run. Just call and bitch and tell them you will send it as many times as it takes until you get your rebate.

I had a problem with Netgear when I bought my first router. When I called them on the phone, they actually sounded surprised when they found out I made copies. Apparently, not many people do so.