Underhand business

I opened a wifi contract with Telkom on 28 April. I had stated clearly that they must debit on the last day of each month. In all my interactions with them. On 15 May 2020 I received an SMS stating that Telkom would debit on the 5th of each month. I don't know who changed the dates or why but I still sent a reply to them, even on Twitter, telling them to please change the date to the last day of the month. The contract was for 10G at R130 a month. Now I received a statement for August which states that they will debit almost R300. When I enquired they said it's because my account was carried over May as they only debited on the 5th of June. Now I have been asking them to retrieve the call recordings and listen to them, they will hear me state that the account should be debited on the last day of the month. I think this is their way of trying to earn more money by choosing their own debit dates which allows them to carry the balance over and then charge extra money. I am not going to pay because that being underhand. I expected more from them. Even thinking of cancelling the entire contract!