MTN charge for PACKET SWITCHED DATA CALLS

My contract was due for an upgrade with MTN. I went to the MTN store in Gardens Cape Town and upgraded to a higher package with MTN. I received my invoice today and I was shocked. MTN charged me R1050 for packet switched data calls. After calling the hotline (got cut off 5 times before) they told me that my data went from capped to uncapped. They told me that I should have informed the person at the MTN store that I want uncapped. Surely the person at the store should have inform me about this. I wasn't informed that my contract will change to uncapped. I'm a customer for many years and this time my invoice is 5 times higher than usual. It's not my fault that your employee is ***********. I accept a credit note from MTN.