Wanting more money after I've already paid

I called and canceled my medical aid. They sent me an amount I paid the full amount. They said the med aid is canceled. Suddenly 2 weeks later I now R331 more due to miscalculation or whatever due to cancelation date. How is this my problem I paid what I was told to pay and after that you want more. You are a bunch of sharks. Get your stuff together and fix this. Why must I pay again. I sorted it out before leaving the country now I need to try sort this out again . How must i do this ? This is your error.