I had something like that happen once. The cashier was throwing my food around and when I got to the car realized that I wasn't charged for half of my produce (due to his rush). When I got home I called the store manager and said I'd like to report an employee that wasn't acting correctly. They stopped me and said "Let me guess, it was Mr. #%$#@. He has been acting up lately." Yes the name was correct (it was listed on the receipt). I said what happened, and as a thank you for being honest they wrote up a $10 check for me to pick up the next time I was in the store.
You can try it yourself. Being honest may earn you money. Of course if they don't care, don't tell them your name and don't pay the $8.
Note: name hidden since it was years ago and I don't remember anymore.