Poor customer service and full refund not given by Takealot

Really bad customer service and lack of co-operation. I purchased something on a credit card for R139 which was not suitable. I returned it and they made a mistake and only refunded R39 to my card. When I raised the shortfall with them of R100 and asked them refund the R100 to my card, they refused and said I had used a coupon to buy the product, which I had not. I tried to explain this to them and they stopped emailing back and would not sort this out for me. Really exasperated that they can give such bad service.