Data over usage

We've upgraded my phone at the end of April, we were told that I can change my contract because my history shows that I never go over my limits - this we did. According to me, if your contract states that you only have a limited data usage your usage will be stopped when you reached the limit and you will need to top-up. In my case this did not happen and now I'm billed for R10k of usage, also if your monthly fees for the past two years were never higher that R800.00 per month why do you only get a notification via email when your unbilled amount is already at R10k - why don't you get a notification earlier when the unbilled amount is still low enough for you to manage and pay. I don't have R10k to pay - what do I do now???? <div><br></div><div><em>(Supplier name changed from Mtn to Autopage by Hellopeter.com)</em></div>