Stores have to pay a percentage each time a customer uses a credit card. Some stores will give you part of that money back if you pay in cash.
I have gotten discounts by paying with cash for jewelry and even for visits to the doctor!
This is something I try to do more, but for some reason it seems embarrassing--I don't know why!
I have never tried it for small purchases, or at the grocery store, but I'm going to try it and report back.