For the rebate receipt: Go to your order history on Amazon, and click on the "invoice" link for your QFR order. That should take you to a printer-friendly page where you can print out a receipt for the order.
For the UPC: You need to cut out the barcode label from the side of the box. It'll be on the left-hand side if you're looking at the box from the front. There's probably two barcodes on the label. I would just cut out the entire label with both bar codes just to be safe. There's also a smaller barcode on the back side of the box sometimes, but that's not the UPC. You won't get your rebate if you submit that. I think it's just a serial number.
Also, before you cut the UPC off, be sure to test all the keys and make sure everything is working. Because once you cut it off, you can no longer do any returns/exchanges through Amazon. Same goes for any mail-in-rebate purchases that require cutting the UPC.
As for wearing down the keys: The only keys I can think of outside the 37 that might still wear out unevenly quickly are the 1-5 number row keys if you play a lot of FPS games and use them to switch weapons. I know because I wore down the WASD keys as well as QER, so replaced them. But even though the replacement WASD matched the rest of my untouched lettered keys, the 1-5 still looked very faded in comparison...haha.
PBT, or light colored keycaps do help though. The stock QFR keys fade pretty quickly.