over charge

I went at game store to upgrade my contract say three month back my billing for the month R209 but the i paid R768 for my first month the i second month i paid R919 i went back to game store and ask them what is happened they told me to call callcenter i called they can't help me cause i have a opened line they told next month I'm going to pay R2350 but the painful thing is i don't use that simcart like the way they say i go on internet .