MTN robbing me on contract amount

Signed contract last year for R39pm top up which includes R50 airtime balance. This was an upgrade from my previous contract of R29pm for the same deal in my previous deal. I was billed R49 since inception and when I noticed this on my bank statement I went in to the store and was told I signed for R49 despite me showing them the document the store gave me stating that I was correct. I have been back and forth many times with no resolution as they are always 'waiting for documents from head office'. My bank statement for this month has now gone up to R59pm for R50 airtime which is laughable. I just require the amounts credited to my account and the contract reinstated to the correct amount as signed. Even today on the MTN brochure the deal is R39pm so I do not understand what has gone wrong.