I'd be surprised if they didn't either accept your purchase receipt, or at least allow you to come back with it the next day after you've gone to the hill (with the proviso that they will charge your card for the rack rate if you don't show it to them before check-out).

I've used this discount in the past, but as I recall, I was staying at the BSH for a few days first, so I had already had a chance to get to the hill to pick up the card before I checked in at CLL.