Mtn charged me Pro rata twice in 2 months now, why?

Mtn is robbing me of my money because of a thing called prorata. I was not informed of that when i went to upgrade my contract because if i knew i wouldnt have done it. Mtn must understand that we plan for all debit orders and i cant afford to lose an extra cash from my account. Ok I just updated my contract with Mtn in the mid September to R149.00 data only on a monthly basis. On the 25th Sep i was billed R320.00 and when i asked they told me i was charged prorata as i took long to upgrade after my past contract was ended/expired. Octomer again i was billed R278 instead of R149.00 and when asked they told me it is prorata again. So how many months must i supposed to pay prorata? Is prorata not supposed to be paid once, please assist. This unacceptable really and even wjen you require explanations from both their customer service and Stores you just cant find it. They seems not to know what to say to you. Can i someone help me get my refund back