Over charging on upgrade to a new prepaid contract

Up until March 2017 I had a prepaid contract which cost me R121.50 per month. I spoke to the Cell C store at La Lucia Mall in Durban North, and arranged to enter into another contract when this one expired. My April account was for R245.57. When I queried this, after much to and fro between the store and the Cell C customer service line, I managed to ascertain that I had been charged the full amount for the earlier contract, plus a pro rata amount for the next contract, which was R139.00 per month. When I protested that this was not right or fair and asked for a refund, I was told that that was the way it worked. I should have been told by the store. I pointed out that I would never have accepted it, had I been told. However, after innumerable phone calls and e-mails I have reached a blank wall, of "that is the way it is". This is really unacceptable. If I could do so without having to pay a penalty, I would cancel my contact and go to another service provider.