I'm not holier than though but I am honest. I would absolutely contact the store asap and see how they wanted to rectify the situation.
One time I was leaving the grocery store at night and thought I may have scraped the car next to me a tiny bit while I was pulling out. I wasn't 100% sure though. I pulled back in and sat in my car for 45 minutes waiting for the person to come out so he or she could tell me if the scrape marks were already on their car or if I had done it. The person didn't come out and I had to leave. I then left a note on their car.
They called me that night and said that they didn't see any damage that wasn't already there.
I also had my car door blow open one time into a parked car. I left a note on their car, they called and I gave them my info and I paid them for the damage. Even if it costs me money, I'd rather do the right thing.
But these things were your fault - I also hit a parked car, left a note, and wrote them a check for $850. But why should the OP have to give up her time and gas, because the cashier made a mistake? If I felt that badly, I'd call, and offer to hand them a check when they arrived at my home.

I nearly fainted when I saw the price!
I dunno, feel guilty,,(?) donate somethng , somewhere.