Dishonest Discovery Health

I received a letter from Tsepo Mariga, the DCOO of Discovery informing me that as my twin sons were turning 21 they would be regarded as Adults and my contributions would therefore be increased. But "If you want to remove (My sons name) from your membership, please let us know by 31 August 2024 to avoid any unexpected costs." But on the 29th August when I phoned to remove them from my policy, I was informed that as I had responded after the 24th I would have to pay the full, increased contribution (extra costs). I pointed out to the agent that this was in contravention of what was said in the letter, but he shrugged it off and said there was nothing he could do because their deductions are sent to the bank on the 24th, so I would just have to pay.

He did mention that Discovery would reimburse me, but how can I trust they will do that if the DCOO doesn't even know what's going on in their departments and send's out letters with incorrect info?