Billed for the wrong package

on the 4th of March I went into the Somerset mall telkom branch where I went to downgrade my 50gig wifi package that I have with them to the 20gig package. First it was an issue as the shop sales man says I must do it via telephone. I phone the call centre in the store. The sales man on the phone says he cant help me i must do it in store. So in the end no one can help me. After much struggle i did fill in the papers to downgrade my package. I phoned the accounts to confirm. On their side it says I have a 20gig package but every month I get billed for the 50 gig package. I have phoned and complained numerous times in store and on the call center and still this is not resolves. Ridiculous! I do not know even where to start to fix this