Incorrectly billed for data

I purchased a 1 day unlimited data bundle from my MTN app yesterday (21/06). I got an sms confirming my purchase and then started my fairly large download (hence the purchase of the unlimited data bundle). Three hours later I got an sms to inform me that I have exceeded my credit limit and my services have been suspended! I went to the Sandton store today (22/06) and the (very helpful) assistant showed me on the system that I had purchased the bundle and then used the a lot of data. However, I had been billed (incorrectly!!!!!!) for the large amount of data!! He also informed me that they cannot log a query on this now as the bill is not closed. I now have to battle for the next 3 weeks with an incorrect overdue account because MTN's systems are *******. This is unacceptable! He also informed me that resolving issues are taking VERY long and that I would probably be billed for this ridiculously large account (more than 5 times what my bill usually is) at the end of July! This is completely unacceptable. Where is the customer service? I am not the one that did something wrong!!