Agai Rain bad service

Rain again.They cannot get my bill correct.I have downgraded my contract to R539 per month.This was their deal at the time.I have paid the amount of R 539 twice.The next month the bill was again more but I was credited the amount that was overcharged.In a DM I have now been informed that the amount I must pay monthly is R 739.I did not sign up for this.Looks like Rain cannot get antthing right!!!