Over billed on my contract

I have been billed normal data rate on a contract that has ended this month, without any communication that I have to upgrade or cancel the contract, they informed me on the 12 that my promo data has ended and that I will no longer pay 260 but 1015, however I payed 330 on the data promo every month. I took out a double deal of 499 a month but my account has been debited 1945, I visited a Mtn store twice with no resolution or clear answers. How do one pay 1015 for a 20G data, that is day light *******.