Karma is nothing but the law of cause-effect or what goes around comes around. It is a universal law and it exists whether you believe in it or not.
This is how I understand it works. Everything in the universe is energy. Energy can neither be created nor destroyed but can be converted from one form to another.
So any action involves using energy. When you use energy it has to be balanced somehow by expending it. In the above scenario, the cashier and the customer has used some energy and it will be balanced somehow by the law of karma....and it also depends on the type of energy used - positive or negative.
My 2 cents...
