EACH MONTH A DIFFERENT AMOUNT DEDUCTION

Can someone please tell me what is the use of signing a two year contract for a certain amount which by the way, you budget for, and then apparently in the fine print , which also NOBODY tells you when signing it , Vodacom can deducted ANY amount they pleases from you debit order - I signed up for R389 per month , and my current deduction just increased to R447.35 and for the past two months another increase to R448.58, what nonsense is this .????? 15.32% increase just like that without any explaining . Vodacom I tell you now , I don't care if you are the last or only provider , I will NEVER make use of your services again