Budget, and a very carefully constructed list.
I will put into the cart whatever my heart desires.
I buy what I need, no more no less. Sometimes if there is a good sale on something, I will make substitutions, or get items that I'll need later.
I do both. Just depends on the day.
I have an Excel spreadsheet where I keep track of the stuff I buy. When I need to shop, I mark down what I need, sort it by store aisle, mark down if I have a coupon for that item, and print it out. When I get to the store, I start at one end and work my way toward the other end, only hitting the aisles that contain something on my list.
My grandparents buy the most of the food at the moment but mostly I buy sweets and snacks myself and then I have tried to keep me to a budget but many times I just buy what I´m craving for.
me, fill up the cart? nope! filling up other people's carts though. and I have to budget while they don't.
Budget. but I do sneak a treat or a few of them if I have the money for them.