Got a 500 dollar check from an old credit card?

Last year I had a CreditOne credit card. Absolutely horrible company that everyone should avoid. I built my credit up a bit then canceled the card after just a few months. AFAIK I paid off the complete balance when I closed the card.

Fast forward to this morning, I get a 500 dollar deposit from them in my account? I'm not trying to avoid calling them because I want to keep the money, but rather I'd like to avoid it because their customer service is dreadful and I don't want to give up my afternoon. What do I do?