Wrong billing due to system failure

I went to upgrade my contract , i asked for the package of R570 that includes 1G data, R200 airtime and the phone. First billing end of November come R2500! Went in and they said no it was an automatic update my phone did, and the lady removed it. Next month came and it was R2300, they said no the system never updated that my contract change!!??? So i payed again and they fixed it again. This moth it was over the R2000 AGAIN!! and the same excuse of the system. So why do i need to pay for there systems OR there mistakes they making. Why is there every month another excuse every time!!!