Increased my interest rate for no apparent reason.

I got a vehicle financed through Nissan Finance - WesBank earlier this month. Last week The Engine room called me to offer me an added service (Scratch and Dent cover) The sales lady Conni assured me it will not affect anything but my premium will increase by R64,15. I accepted and later that day I got the documentation and noticed my premium had actually increased by R181,28. I investigated further and I found out that the added service had increased my interest rate by 0.013% thus increasing my principle debt and the premium. I spoke to The Engine Room and I was assured that this was not meant to happen. I called WesBank finance yesterday and I was put through to a guy called Tshepiso, he was rude and unpleasant to deal with. He told me that the interest rates always fluctuates and I told him when the interest rate changes the whole country knows.

I would like to remove the added service off my plan as it has just brought complications, I would like my interest rate to return back to what it initially was before this and just to be left alone.