Get yourself a Unit Price Comparison app for your next shopping trip. It will let you know how much you're paying per oz/lb/kg/ml and tell you which is the better buy. A good app will let you save those prices for future comparison.
Except that for some products they show the unit prices differently for some maddening reason. Like $.04/ounce right next to a competing product that has the label $.40/biscuit or something similar. Like they are trying to make it more difficult for the shopper to compare so they throw up their hands and buy whatever they are used to. I see this frequently.
Thankfully, in France that's not a thing I ever came across. It's almost always SI units when applicable or occasionally, it's missing, especially when there's a sale (supposedly) with a custom tag to attract attention.