Complain

I would like to place a complaint regarding my pep account. Every month when I pay the is a hundred Rand that's been added when I call and ask they say it's an interesting fee and card charges .but due to my own understanding the amount u given to pay every month end it's supposed to be including the interest fee.so my question is how do you finish to pay a loan when the interest fee it's been added every month when you have paid ?