Incorrect collection amount

I had two cars with their tracker installed. I sold both cars, a year later they were still billing me. I blocked them, accumulating R1600, and eventually with Hellopeter intervention they stoped billing. Due to the hassle of recovering what they owe me, I decided to settle R1600, which I paid for 3 moths to R1000. But they handed me to CreditWorx to collect R1600. I request CreditWorx to fix the outstanding balance, sent them proof of payment., instead they have now app**** interest and the amount is R1900. Why is Netstar operating like this?