Suspicious hidden cost and deduction

Intex advertised a chair in brown and grey in their online shop. I ordered two grey chairs and paid both the cost and the delivery fee. I received confirmation of my order. Four days later, I was informed that the grey chairs were out of stock and that the shipping fee was R520 more. Since I was not interested in brown chairs, I asked for a refund. I was kindly informed that R260 would be deducted due to merchant service. This is not allowed according to the Consumer Act. I would not recommend doing business with Intex.