Breach of our contract

I made a mistake when I used this company to finance my vehicle. They made a mistake with my account and I emailed them. I am getting no reply from them. I signed a contract with a fixed interest rate, however, at the end of the month they increased the amount I pay. They also added an amount that says Premium Debt? I have no idea what that is. It feels like it is a different problem every month when you use them to finance anything. Last month I got harassed by Liquid Finance. Apparently they are WesBank. I first got an sms that they will automatically charge me more for insurance because I wouldn’t answer the Spam numbers. I proved that we have sufficient insurance and they still said that we don’t have enough. It was only after my husband sent an email that I received a message that said that everything is in order. I just want to pay the amount we agreed to and what is written on my contract. Could you please stop creating problems for me at the end of every second month. I hope that you will fix your problem and pay back the extra money you took from me at the end of this month. I want to buy a car again, but I can’t use a company that just creates problems.