I work "in a grocery store" or "at a grocery store" [duplicate]
Solution 1:
They are almost interchangeable, but you could convey a subtle difference in meaning. If you're trying to describe your job / what you do, you'd want to say you work "at" a grocery store. Working "in" a grocery store describes the location you work at. For example, I work in an office, but I work at a company.
Solution 2:
AmE here.
I would use either interchangeably in speaking without giving it a second thought. If it were important to be precise, I would simply state my position (I'm a cashier in a grocery store, or I'm a cashier at Foodmart.)