Double Billed

So i took out a contract last year June and ever since then i was being billed incorrectly. I took out the contract for R189 and every month my amount differs. I queried this matter telephonically and they said that it is self service charges, which i do not understand. I get a R200 cellphone allowance at work and every month this far it differed. With this R189 contract i receive R60 airtime and i top up my account as well. I am being double charged and took screenshots each and every time i purchased anything as i select it from my airtime. They said i should change my amount to R0 but then i couldn't purchase any data or minutes from my airtime which it shows normally on top how much is left. I do not purchase anything more as i have a private cellphone as well and only use this phone for work purposes but then the amount differs every month and my airtime disappears. I am currently keeping track of my monthly bill from July that i have been over charged for. Why would i purchase on my account if i do have airtime and if i purchase anything i select to be deducted from airtime option. I want this rectified as this is ***** and i am being double charged. It gets taken from my airtime amount and billed every month.