No. The majority of vegetables I buy (in Canada but it's the same in the US as far as I've seen) are not pre-packaged. Self checkouts have a scale built into them that you weigh your own produce and punch in the code to say what kind of food it is. It's trivial to cheat if you want, but most people are honest. This is also true in hardware store self checkouts where you buy loose items and the store relies on you being honest about punching in the correct code when you check out.