MTN is *****ing from me

End of February my contract ended during that time received a call from MTN to upgrade, of which I declined as I had financial constraints. I was offered a deal to continue paying for data, minutes and sms for R129 monthly To my shock up to this date they have been taking from my account R329. For the past four months I have been trying to call them but they have been taking me from pillar to coast. When I went to the store they told me that they can't do much as the deal was done on line. Please help me to get my refund back. Thank you