Not interested in helping

I bought three tins of chopped tomatoes which where on special - buy three get cheapest free. When I got home I noticed I I had been charged for all three. So, I went back to the store to ask if they could refund me. The cashier rep**** that she'll have to ask the manager, who arrived after a VERY long wait. She asked if I had brought the three tins with me. I had not, but I did have the till slip, which listed the items and showed that I had paid for three instead of two, as advertised. The manager, however, refused to refund me. Not only was it an inconvenience to have to return to Woolworths to rectify a mistake they had made, but I was also treated very badly when I went to do so. Instead of helping me, the manager was rude. I expect better from Woolworths stores.