****s masquerading as a tracking service

I cancelled my car track in April this year, Car track said I would them R600+ okay. This is because I sold my old car for a new one that came fitted with the tracking device. Cool,. I asked that Tracker send me a copy of the contract and explain how they came to this amount. Nothing! Few weeks later they debited R759 from my account to which I disputed because it was not authorized. Last month Car Track debited R37.50 on 26 July 2018 and additional R236.53 without my authority. I did not reverse these. Since then I have been harassed by phone calls and "letter of demand" but they have failed to explain to me how I owe them when I ended the contract. Where is the contract that clearly the cancellation fee?