Cashier should listen to her customer

I was at the Eastgate Checkers Hyper, I explicitly said I don't want a plastic bag when the cashier asked. But she rang up a plastic bag that I never received, I only realised this after I left the store. A bag is only 50c but it is a matter of principle that not only was I charged for something that I didn't want, I didn't even get the bag I had paid for. You need to train your cashiers to listen to their customers.