Uh, I would call them and let them know. Make them pay for all shipping.
I had an incident at AONW, i picked up a gun from a repair for my cousin, had a battery with it, asked about the battery and they thought it went with the gun. I dropped it off with my cousin and he said the battery wasn't his. I called and since it was a 2 hour drive back, i paid for it over the phone, I did get a nice discount, and a really good battery too.
Honesty is the best option. I would fondle the gun for a while before i send it back tho
