got a FREE apex N1

[nlzmo400r]

ok, heres the deal: a friend of mines mom has a paypal account that i always use to pay for my online orders. I ordered an apex N1 exhaust from an online company, however after about a month of waiting, i was told it was on backorder. So i canceled the order and got a refund via paypal to her account. About a week ago i received the exhaust anyway. My friends mom says i should 'do the right thing' and give them the money'. I say its a free exhaust, and if htey ask for hte money, give it to em. But she wants me to contact hte company and get it straightened out. I just wanted to know if this had happened to anyone else before and what they did about it. What are my options? Thanks in advance- Rob