You know, I actually find your honesty refreshing... I know most people would brag and gloat that they got something free without any guilt... but you want to actually go back and return it! Wow, that's cool!

Yeah, definitely give them a call and let them know that they didn't charge you for an item, but from my experiences, they certainly won't make you go through the hassle of coming back to the store to return it unless you REALLY REALLY REALLY INSIST upon it lol. They'll probably just thank you for the honesty and ask you to keep the item! And I'm sure that would make you feel much much better!