It seems that by asking advice you already know the right thing to do. I'm with ky and LJ on this one. Had a similar thing happen when I bought a kayak. The girl screwed up on the bill, charged me and then credited me. I went home with around $2K worth of boat and gear. Didn't realize what happened till two credit card statements later. I went back and told them about it. They had no idea that they were out the money but were very appreciative that I was an honest person. The manager wound up charging me for everything but the $150 paddle as a thanks.
01 Jeep XJ w/ 200,000 miles as of 3-26-11