Wrong order delivered twice

I had ordered three dresses and one of the dress which was a backless ankara was not what I had ordered, which I had returned and they made me pay R100 for their mistake. Today, they sent me back the same dress different size and it's still not what I ordered and I'm guessing I have to pay another R100 for them to come get it. This particular dress is R250, meaning I'm going to pay R450 for their mistake. All of this is happening even after talking to their centre agent and writing them a note an inserting it in the package when returned back. How is this good business practice? How competent are you as a store?