I had the same thing this weekend - I was running in and out of the (overpriced) souvenier shop to get the girls some trinkets from the snow and then discovered I had picked up a free item as the assistant hadn't charged me for one of them. I was trying to keep the girls hands off the merchandise at the time so didn't check the total and didn't realise until it was too late. There was no way I was going back up the mountain for $20 so I figured it was just one of those things I'm sure I paid more than enough for everything else on the mountain to make up for it.