Cartrack is a ****

I took a car tracker with Cartrack. They debited their R120 on the first month. Weeks later they sent me a WhatsApp saying I owe them R109 I inquired about the R109 and I was told what it was for, I paid and sent them proof of payment. My issue with them now is that they keep sending me the same WhatsApp message saying I must pay the R109, I keep forwarding the POP to no avail, when I ask them how many times must I pay this R109 they don’t respond but keep sending me the same WhatsApp text.