I Overcharged That Customer

So this happened the other day at work. I was running a bit behind in my cashier duties because I had to help out with some inventory issues earlier. A customer came up with a cart full of items that needed scanning, but their receipt from another department was confusing me with some incorrect information on it. In my haste and slight frustration, I ended up charging them for an extra item that wasn't even in their cart. I noticed the mistake about five minutes later when I had a breather. The amount overcharged wasn’t huge, just a couple of dollars, but it felt like a mountain to carry around all day long. Of course, I could have gone back and corrected it right away, but there was this weird sense of guilt that made me hesitate. So here I am, still feeling bad about not taking care of it yet, it’s like having an extra task on my list that nags at me every time I pass by the customer service desk.