Well, after a while I do roughly memorize it for the things I buy the most, don't you roughly remember the non-per-unit price if that is what you are looking at

Well, after a while I do roughly memorize it for the things I buy the most, don't you roughly remember the non-per-unit price if that is what you are looking at ? But that was not my point, the price per unit is to compare similar products with each other in the span of a minute or so. Because if they are ALL going up (or down), then there are clearly external factors to that price change. At worst it's the fault of the supermarket (and here remembering the rough price/unit comes handy, for comparisons in others), or an issue with a monopoly/cartel collusion, in which case hopefully consumer associations are going to start whistleblowing, leading to government handing out punishments.