How can anyone possibly know they're being over or undercharged on an item? Unless it's a "special", do all of you know the prices of every single item you buy?
Right?
There's no way I'm remembering the exact price of every item I purchase on a regular grocery trip. I don't even know the typical price of items. I usually have a list, & I'll shop & price-compare from my list. But I'm not trying to remember down to the cent what each item costs. I don't know if the green beans cost $1.79 or $2.54. But, this week, Del Monte was on sale, so that's what I bought.
Now, if there's something like a "buy 1/get 1" promotion, I'll remember that & try to make sure it rings up correctly. Or, if I've purchased 2 of something, & the cashier rings up 3 of the item, if I notice it, I'll say something.
The other day, we were at Target, & I purchased 2 packages of mini-candy conversation hearts & 2 packages of regular conversation candy hearts. I noticed that the cashier had rung up 3 packages of the 2 mini-candy hearts instead of just 2, so I said something. And, then on the way to the car, I noticed that she also rang up 3 of the regular conversation candy hearts at well, but it wasn't worth it to me to go back inside. But I couldn't tell you the actual price of the conversation hearts.
However, if I'm buying particular items (a piece of clothing, shoes, a frame, whatever) & the items aren't grocery-related, I'll typically take note of the price & try to make sure it rings up correctly - like, "Oh, I thought those shirts were 25% off?"
But, as to the original question, I think it would need to be over a dollar at least. And, if I notice it after I get home, it needs to be an amount that's worth my time going back.