Sounds like you got a good deal. I've dealt with Backcountry Gear (a lot), and they're super. I've sent a couple of things back, with the hang tags intact, and they never batted an eye - full refund, promptly executed.

If you really want both packs to try, call them tonight or first thing tomorrow - they might still be able to add it to the order. I've done that, too. However, with their free shipping option, your plan will also work - and maybe save you paying for shipping on the return.