Poor Service provided.

I've been overcharged in one of the stores(Bethlehem, Dihlabeng Mall) when I call the store they tell me I need to go back to the store tomorrow as their promotion is ending. Is it my mistake that now the cashier decided to charge me for one item 3 times? I live 80km from the store. Their service is disgusting and to think that whoever I was speaking to was trying to hide it.