“Out of pocket at their mistake !!”

I purchased a product on 4Home.co.za and paid for it online. I received a call the next day to say that the product I purchased price was advertised too low and that I needed to pay more for it. I felt sorry for them and asked them kindly to refund my credit card as I now needed to buy this product from another supplier, but I explained under normal circumstances that I was entitled to receive the product at the lower price as it was their error. They said that they would refund my card immediately. After 2 weeks I had to email them and ask them why they had not refunded my card. They said that it’s in the process. I have just received my refund almost 3 weeks later but less then what I paid. I am now out of pocket !!! Such bad service and dishonestly. Why should I lose money when they made the error in my favour with their price in the first place !!!