There are products in US that are in nice metric sizes and weird US sizes. Like the 2L soda.

Why don't you just look at unit prices instead? That's the only price I ever look at and other than edge cases like trying new things or goods that you can't use up fast enough, there's no reason to look at anything else.

If your store doesn't have them, that's a sign you're at the wrong store and everything is overpriced.

Teach your kids this

PS: Walmart, which has pretty good prices, is famous for odd prices everywhere, so your heuristic would fail. All in all, really, a heuristic like this is a pretty weak way to try to be frugal and I can't recommend it. Just check the unit prices and shop at Aldi. Buy commodity ingredients, not trademarked factory food products. You can't help but spend a fraction of what other folks do when you just buy commodities by the unit price at a store serious about prices.