This might be a tough concept to wrap your head around, but Amazon's new "Go" store in Seattle doesn't have any cashiers. It doesn't have any self-checkout lines either.
Basically you walk in, scan your phone while the Amazon's "Go" app is open, take what you want off the shelves, then leave. Amazon's smart technology knows what you picked up off those shelves, so there's no need to take it to a cashier or self-checkout line to have it wrung up. You just leave, and Amazon charges you for what you took accordingly.