Bad Customer Service from Cartrack

I'm very dissatisfied with service I got from Cartrack, I have been with them for a while now and always was on debit order but recently after I traded my car asked them to come and remove the device from my old car and install a new device to my new car they told me it will be 870 which I had no issues with paying the amount and gave them a date when I'll settle that but since then I have been getting WhatsApp's and emails regarding my account being behind and mind you they are not referring to the 870 it's a different amount each day but I have never changed any payment method and bank account remains the same I even went into the extend of sending them a statement from bank account but I still get ridiculous messages that I have made arrangements on the 25th which is absurd I'm tired at this point I'm thinking of taking legal action as I have requested that they send me proof of which months I'm owing but nothing! I'm done explaining over and over again.