Mtn extra time

I have been using mtn extra time in emergencies. I borrowed R15 and asked if I accept to pay back R17.20 which I agreed to. I was then charged R22.20 Then after numerous emails I was told my subscriptions took my airtime and I should cancel it. I have no subscriptions. Then just to check I borrowed R33 and was told I have to pay back R37.20 but was charged R44.20. And still they have no explanation for me