Bad Accounting/Billing

I have a tablet on contract with Cell C and receive 2GB of data monthly for R 600. We never use the data and it accumulated to over 3GB in December. By the end of December 2014 we checked to see how much we have left and to our shock we saw that we had over 2GB left, but our current usage was R1,900. Upon phoning Cell C, they told me that as soon as we download thing exceeding 10MB, it does not use the data but it starts charging the account. So what is the point of getting 2GB of data every month. Prior months I used the tablet to download movies, which is well over 10MB and never had this issue. My total bill came to R 2,500, which I asked to pay it off monthly, seeing that it will be expiring soon. After I was put on their system as an Exclusive Customer, I have been struggling for almost two months to get confirmation that I have made payment arrangements. They keep debiting my account and I will not pay this in full until they have investigated. The same thing happened on the contract my daughter uses. She gets 1GB data and by the 3rd days of the month it was all gone and they started charging the account. Why bother having contracts if you still end up paying so much Rediculo