You bring up some really good points... Money is spent on more than just actual food. Most people do not take aspects like you pointed out into real consideration. If you go here, it says that the food boxes will contain

"USDA would utilize a model similar to that currently used to distribute USDA Foods to other nutrition
assistance programs to provide staple, shelf-stable foods (such as shelf-stable milk, juice, grains, ready-to-eat cereals,pasta, peanut butter, beans, canned meat, poultry or fish, and canned fruits and vegetables) "

And they would still receive some food stamps, so they still will be able to buy other items at the store.