Free shoes

Hi I went to Mr Price yesterday in Cape Gate Mall. It was super busy and I hate going to the malls this time of the year but I needed shoes. All I bought was 1 pair of shoes and the plastic bag, which My husband paid for with his card. The lady at the door also checked the receipt and stamped it.. when we got home, we saw on the receipt that instead of charging for the shoes, they charged for 2 plastic bags. Now I have the shoes which we didn't pay for. I do not want to go back to the mall. What am I supposed to do now.