Amending contracts without customers knowledge and consent

I have had a contract with vodacom for years and this is the first time I encounter this. I wanted to cancel my contract with them and they offered me a discounted deal for 24 months I accepted it and all has been well till they decided to increase it to R300 which wasn't even the initial contract price (R299). They decide to extend my contract I have no issues I am still paying the same amount. They again decide that my discounted offer should end and they put in a new contract without my knowledge or any form of communication. They don't even bother to ask whether I am ok with it or do I want to cancel it. They make decisions about my money without my consent. After I complained about how this was handled this their response to my issue

Good day dear client

Please be advised that as per the upgrade deal: *** , the discount of R159.13(exc vat) started 06/07/2018 and should have finished 06/07/2020 but extended until November 2020 hence you only receive a partial discount of R153.83(exc vat) for ***.

Please note the discount has now expired and you are liable to pay the full subscription.

Yours sincerely

Wouldn't it be nice if such was sent before they decided to bill my account without my knowledge for R300? Contracts are legal agreements entered by more than one party and when cancellation or amendments are made all parties involved should be in accordance. I didn't agree to R300. Please balance me here and make me understand why I am liable to pay for something I had no knowledge of and never given the opportunity to accept or decline.