Congrats if you are able to do so.

I often try to predict the value of the items in the shopping cart when waiting in the cashier queue. Most of the time shopping in a supermarket, I can predict the total sum in a range of +/- 2 Euros. My girlfriend? Perhaps in a range of +/- 5 Euros. My mother? She would only be irritated if the sum is at least 10 Euros above what she expected. And that is for 15-25 items and a total sum in the range of 40-60 Euros. But there is no way my mother, my girlfriend or even I would detect small over- or undercharging most of the time.

It's not about the total, but the prices of individual items as they show up on the checkout screen, which are pretty easy to remember (they're mostly the same week to week).

My local supermarket has some specials which are loyalty card only, so when the item is scanned the full price shows up. I use this to determine when to use the loyalty card for minimum tracking, minimum time, and minimum price (it doesn't have my personal info associated with it, but still connects purchases and it's faster to skip).

People with a good enough memory. Unfortunately, those most in need of watching prices are under a tremendous amount of memory-obliterating stress.