Overcharged on data plus attitude

I have been a Vodacom client for more than twenty years and had always having a 24 months contract. As per the norm, I made an upgrade from talk 500 to Red advantage on 17 January 2015. During this time I was informed that my contract will stay the same till the 01February 2015. I then continued using my new phone as I had BIS and I didn't buy any bundles as my contract stay the same. On 25th January I received the from Vodacom warning me that my account usage is high and when I call them (customer service) they told me that my data usage is too high. During that time, my usage was just over R900 and I bought data (250 MB) to cover the remaining days of January after I was told that BIS is not covering me. To my dismay, my account was charged over R3500 of data for the January month. How did the amount come from R900 to R3500 whereas I bought data when I found the problem. Remember that I went to Voda shop during the week because the 25th was over the weekend and they also confirmed the R900. I then complained and the person by the name of Cheryl Reid sent me the response to say only 50% of the cost will be cut.