it will get better as she understands more.

with our ds we told him to hold onto the trolley or a hand and if he doesn't its straight back in the trolley tantrum or no tantrum
DS also gets a snack, unfortunatley these days it's has to be something special - but the younger they are the easier it is to get away with fruit or a sandwich.
I've heard tips for older children being giving them a shopping list of things they have to find. by about age 2 you could try a picture shopping list, stick pics of food out of the catalogue on a piece of paper and she has to find all the things on her list.

Recently I have had the most luck with ds in the produce section of the supermarket where I ask him to pick out all the vegies. he's getting goo, i think he knows more vegies than all those american kids on jamies kitchen already.