Canceled my contract then they want to charge me more then double compared to my previous month's .

After 2 years I wanted to cancel my contract I was paying around R300 a month because I got a discount . So first they wont let it run out I must cancel that's fine but then I must give a 30 day notice. So I assumed pro rata since I canceled mid month no they forcing me to pay a full extra month. Then I received my last invoice and it's now over R600 because since I canceled my contract even after two years they feel like I should not get the discount any more. Cell c forces you into a last month where there can charge you whatever they want.