It's just about impossible to compare prices in the US because it switches between oz and gallons or oz and lbs.
It's pretty unusual to have the same product measured in two different units. It's also pretty unusual these days for the grocery store not to have a price-per-unit sticker. Finally, it is unusual not to have metric printed on the same label, right next to the standard units. Milk gallons have 3.78 L printed on them (or stamped into the plastic). If you end up here again, just remember that it's 16 oz to lbs or pints. 2 pints to a quart. 2 quarts to a half-gallon.
I'll admit to breaking out the calculator from time to time, but usually because the amounts are an odd amount (e.g. 333g or 11.75 oz). The price can also be any weird decimal, which can make the mental math difficult.