BP Big tree mall terrible service

Driving back to Johannesburg on Monday i stopped at the new BP on moloto road at the big tree mall. Went in to buy a packet of 10 staveysent blue. The cashier gave me a wrong pack. Realized that before leaving the shop and went back to correct that. Her response was that they do not exchange items that left the shop already. I told her that i did not even leave the shop. Then she still insisted that she is not exchanging it. Terrible service where people think that they are doing you a favor when you are actually paying with your money. Disgusting attitude from the cashier.