Incorrect debits and no attempt to correct it

Really frustrated with Discovery. At the end of June I took out the new warranty cover to be activated from 1 July. Then I was debited the incorrect amount, nearly R300 more than what is in my policy document. I queried this and it took almost 2 weeks to get an answer. The answer was that it was a prorata amount for June. Then I asked why was there a prorata amount when the cover is from 1 July. The response was it's because the agent who processed the warranty cover put the start date as 1 June. That's it. No attempt to correct this or refund me. I am just expected to accept it this error. I took out the cover at the end of June so why would I pay for June if I was only going to be covered from July??? Anyway I have been trying to cancel the cover for over a week now because I am fed up now but I can't get through to them on the phone or by email. Nobody gets back to me. I understand that due the pandemic companies are working differently but this is really terrible customer service.