There's a farm here in Zurich where we buy a lot of our groceries. The store is unmanned. You use their calculator (or your cellphone calculator) to add up what you're buying then use your phone to scan a QR code and pay.
I know a few farmstands here that operate on the honor system. Put $5 in the box, take your bag of apples. You have to go a bit out of your way, but they do exist in the States.
+1, every time we hike up a mountain and there's a help-yourself-fridge/shelf, we take 5-10 items and it's too much to add up in ones head. I use the phone calculator and pay by QR code.