I believe picking the right target is as important as executing the approach, keep in mind however that age group is out of range for me since I passed my mid 30s.
If you have a look at the items in her cart you can asses more or the less correctly her living conditions and environment and act accordingly.
If there is alchool or chips she either is a party type or shares her house with someone while if there is only healthy things and vegetables she could be working to make herself in shape and attractive but also buy stuff for kids.
You have to asses it from her grocery cart, her clothes, her overall vibes.
Regarding the approach, it doesnt need too much effort from your side since at that point it's just a matter of raw attraction so just say anything related to a product you're both buying and see her reaction.
As always long interesting answers show interests and short closed answers dont.
Hey that great yogurt is amazing but cant find the strawberry one, did you see it?
a) No I dont, I just buy this one (she walks away).
b)Do they make a strawberry one? Im used to this but would like to give it a try...in which part of the fridge is it usually?
Regarding the women working there, it's a total different game, you basically have some small chat with them everyday and see if they come with personal questions and what kind of questions are those.
They know what you usually buy so they already know a lot about your life style and economy.
Regarding opening with compliments, I strongly disagree with that and I explained
here why, see point 1.