Over Charged and Does not Refund

I opened an account with The Courier Guy about 2 Years ago. I was told that I have a "One Rate Account" I will pay R95 for up to 20kg. Suddenly I get charged between R155 to R315 for 5kg+ parcels. When I query they tell me that I selected the wrong option. We always ask the call centre to choose the cheapest option. I have lost about R2000 with The Courier Guy. They refuse to credit my account. They always just shrug their shoulder and never refund my money or credit my account.