Also, Costco isn't always automatically the best price per unit anyway. Especially for staple items, I find that Fred Meyer (Kroger) is frequently less expensive on a per-unit basis for really common household items. It's deeply ingrained that buying in bulk must be cheaper, but pricing in grocery stores isn't always that intuitive.

Don’t know how it is in America but here in Norway, stores are required to list per-unit price on the shelf for all articles. Makes it quite simple to compare stuff. So for example milk has the price for a carton on the shelf along with the price per liter, toilet paper has the price for a pack along with the price per meter, detergent has the price for a box along with the price for the portion required for running one machine, etc etc.

This makes it trivial to spot cases where the bigger bag is actually more expensive per-unit when you’re at the store.

I guess to compare between different stores you’d still need to make note of the per-unit prices that are listed but still it is helpful even if you went as far as that. I usually buy everything I need at the store closest to my home because they are very competitive on average even if one or two items might be cheaper downtown.

I am not certain what the law is, but stores here in the U.S. routinely list the per-unit price on the shelf tag. Costco certainly does, Kroger does, but I don't know if all stores must. I wouldn't be surprised. I don't often look at per-unit when going between stores, I more often use it to gauge the value of upsizing at the same store, or comparing to other brands of the same product that are on the shelf nearby.

Costco shoppers know that they don't always get the 'best' price, but it's a good enough price and it's likely to be good quality, so it takes the hassle out of the shopping and decision making process.