A few hours later, when you're cutting them to size, you realize that you actually have four of them. You're confused for a moment then remember, "Oh yeah, I grabbed an extra in case I mis-cut one." Now you have four pieces of backing board, but you only paid for three. Backing board doesn't spoil, and you'll use it eventually, so returning it doesn't really make sense. Plus, it gets a tiny bit beat up in transit.
The store is a national chain of art supply stores, and one that you do a lot of business with.
Do you go back and pay for the item?
Yes, if at all possible
Yes, if it's convenient
It depends (tell me in comments what it depends on)
I forgot to buy something, so I went back to the store today. When I got to the checkout I handed the clerk my receipt from yesterday and said, "I paid for three of these, but actually grabbed four. I'd like to pay for the other one now." He seemed a bit surprised, but happily obliged.
I'm not sure what I would have done if I hadn't had to go back to the store today, or at least been in the neighborhood. If it had been an expensive item I definitely would have made a trip back. If it was something that was only a dollar or so I probably wouldn't go out of my way to pay for it but would do so if I had to go to the store for something else. I would make more of an effort for a mom & pop store than for a large national chain.