Good luck on the MYOG pack. I'm finishing up a guide for the latest one I made today hopefully. I like the MYOG packs so much more than anything I ever bought.
As for returning to REI, like Terry said, if you are a member, they will have the purchase saved. It doesn't matter if you aren't a member though. I know my buddy has returned items (using the mail return, not in store) to REI without a receipt before. In fact, I think he accidentally returned something he didn't buy there once. However, if you don't have a receipt of some kind, they will only give you store credit. The bank statement may work, but I'm not positive.