upgrade for top up biling

it all started when i took an upgrade with mtn. i preordered the Sumsang S6 over the phone, i Dealth with a guy called Devashni from Durban based call centre. i requested top up 100 billing plan but ever since i have been paying way too more than i bargained for. i called the call centre many times to try and get my bill standardised to the amount that i agreed up on when i spoke to Devashni but to no luck, i even went to the walk in center in 14th Avenue but i was sent to the shop in clearwater mall and the told me they had fixed the problem. ecerytime i speak to Mtn people they tell me the problem is sorted but shock me when they debit from my account. i called on wednesday the 5/08/2015 to request my itemized billing and they telling me they dont have the information available they still gonna 'generate the information ''