Incorrectly billed after purchasing contract onlin

In October 2015 I app**** for a iPhone 6s on MTN's online shop - the advertised promotional price was R349 pm and I thought this was a great deal and only R100 more than my current contract with more data, albeit less talk time. I app**** and was approved as an upgrade, and was quite pleased with the service until receipt of my first invoice which was much higher. On querying it (OCT 16)they advised me that it was correct and a whole of of other excuses like contract fee blah blah. Earlier this year i came across the contract application confirmation email which confirmed R349 and i started the query process again, still no concrete resolution. they give me a whole lot of reasoning on the phone and no solution. i know what i bought, i know what the offer stated and trying to get a response is a mission and a waste of my time. Surely if they check history of promotions they will see - also advised that the additional charge is my subscription fee, but when something is sold at TOTAL pm,no fine print, then i surely am right in assuming that i will pay that and not more per month. MTN really is grating me - i dont want the hassle of changing SP's and would like credit and rectified