Extra unknown charges

I took an MTN Cellphone contract online and I also too insurance aswell. According to my calculations I was suppose to be paying +-R 1500 but I got surprised when I received an sms that I will be paying R 1852.05. I called MTN and I was informed that there’s extra charges that were added. Why I was never informed about the charges? My budget is R 1500 not more than that