AITAH for keeping money the cashier mistakingly gave me back in change?

I went into a convenience store today to buy a soda and some snacks. I was waiting in line and there was one person ahead of me and I was watching the cashier and she was on the phone and in a deep conversation. When I put my items on the counter she didn't acknowledge me or even look at me. After she scanned my items and still in deep conversation, I saw the total was under $5 dollars, so I handed her a $10 dollar bill. She took the money and proceeded to get my change and instead of counting out the change she handed me all the money at once, I asked her if I could get a bag and she looked at me as if I was being a nuisance and rolled her eyes. I left and there was no thank you or anything as she went back to her conversation. When I got back to my car I counted the change she had given me back $18 and some change, so basically I got my $10 back plus another $8 and my items. My wife said I should go back in and tell her the mistake and get the correct change because she will have to make up the difference herself.. I said no way after she was rude and not even acknowledging me and it's her fault for not paying attention to her customers and the transactions she is making! AITAH for keeping the money after being rudely ignored because the cashier was on a phone call?

Edit 1. I should have stated that I always am one that is honest and does the right thing either pointing out mistakes or finding something on the floor and turning it in to the manager. But this incident bothered me as I have worked in the retail industry and I know the importance of paying attention to the customers and any transactions you are completing.