Incorrectly debited

I started march off on a 10mbps capped line, by the middle of the month my 75 gig day time cap was finished so I couldn't get onto anything that uses Internet!! so I called mweb and asked them what was going on and they told me that I had reached my limit, so I decided to down grade to a 4mbps uncapped line which to my surprise was active within an hour! so as a happy customer I continued using my Wi-Fi on what I was told, I would be billed prorate! I wake up today to a message from my bank saying that I've been debited R2219 for the month of march! so I called them and asked what was going on and they told me that I've been paying per meg I used over the 75gig cap! I don't understand how that works as I was not notified that this would happen if I reached my cap and nor could I actually use my Internet so I don't know how I can reach a bill like that when I couldn't use my Internet and was already on the 4meg uncapped package?? after calling this morning I was told someone will be calling me back, but to no avail. very disappointed customer and will be cancelling my account with them and will never suggest them as a service provider.