Edgars over charging on Accounts with no explanation

Edgars has been refusing to give me an explanation on an installment amount that is exorbitant for over a month now, i have spoken to several of their accounts staff and a supervisor Beverly, with regards to an installment of over R900 on an account that according to Edgars welcome letter should not exceed R801, i have not made any purchase on my account yet my installment amount has increased from a monthly rate of between R690-R750 a month to over R900 with no purchase. Staff is unable to explain a simple amount which is calculated by Edgars as my installment they are all giving me a bull **** story that the installment is correct yet they all can not provide an explanation as to why the amount is so high and how it was reached.