Foschini Account - Nobody can explain the amount charged!!!

I have an account with Foschini (TFG) for more than 10 years and never missed one payment. I only buy in the 6-month interest free plan. In June this year I realized the amount charged was higher than expected (I have a strict control of what I buy and my monthly payments) and contacted them to inform they were overcharging me. After many emails back and forth (with them insisting on the amount) I requested that they send me a report of ALL my purchases for the last 2 years. With the report in hands I added up ALL my purchases (bought on the interest free plan) and ALL my payments to them and it turned out that I have overpaid them in R791,70 in total over these 2 years! But they still insist (and phone me EVERY DAY) that I own them R446,72 (now with interest rates, and SMS, emails and col letter fees charges). I have sent them countless emails asking them to explain how I can own them if I have paid more than I bought (on a "interest free" plan) and nobody gets back to me with an answer! I have phoned the customer service center only to have the line cut off 4 times in a row, with nobody calling me back. I am just in awe of the lack of organization and respect for their clients. On a daily basis I have to repeat the story to a different person that calls me and almost beg them to sort this out and all I hear are threats of sending my name to the credit bureau. I have never been more disappointed at a company before and would advise any person considering having an account with Foschini: stay away from their account!