RCS IS RIPPING PEOPLE OFF

I have a RCS card and i have had since last year and paying it regularly ,but this months i decided to go through my slips that i pay with , i realized that every time i pay my account ,my balance only goes down with about R150.00 and i normally pay a minimum of R350.00 at least ,last week i called and requested for my statement to be email from the inception of the account but the lady i spoke to was rude and told me that its because of the interest, i dont understand how can a balance of about 4000.00 with an installment of around 300.00 accrue an interest of R200.00 .RCS is ripping people off and i want people to be aware of this im a pensioner and it means by the time i finish paying off this account i would have paid and interest of about 60to 80% of the initial credit approved