When I shop, I look at the quantity. If my chocolate has 90 grams instead of 100, I'll immediately put it back and look for another brand - because it's too hard for me to compute how much I would get swindled of.
I'm not sure whether it's a practice everywhere, but many shops in NYC also indicate the "per unit" price: per gallon/oz/pieces etc. I use it all the time, and I wish it was mandatory by law for all products.
I live in Spain and when you go to supermarkets, all price tags on the shelves and on supermarket-packaged products (pre-cut meat, for example) indicate the cost of the product per unit (big numbers), and per kilo (small numbers), so, if you want, you can quickly and easily compare a package of, say, 136g of meat with other packages, without weight interfering. You can see easily the price per kilo of each kind of meat.
