Car Service plan

Good Morning, So after I took car finance in Feb this year, I got service plan offer from Fnb and I chose one for +-12 800 for 48 months. Now I receive a letter of notification of change of installment and addition amount on my installment is R600, when I look at my car finance additional interest is +-2800.I called and got through to Dineo who I explained to her my concerns and advise her to check before I cancel a and all she said was do you want to cancel and asked her if she understood the reason for my call and She hang up. According to my Calc it's supposed to be +300 not R600..