Reinstatement, Oh no sorry my mistake - USELESS

I have never experienced such unprofessionalism and passing the buck. My medical aid was reinstated on the 3rd of April 2019 and I received a reinstatement certificate. From the 12th June 2019 Discovery advised that they made an error in my reinstatement and they apologize for the mistake. The catch, unfortunately, is although they made the mistake I have to pay the calculation error. The worst part is that they advised that I should make this payment as I have had the benefits like I forcefully made them reinstate and make this mistake. This is not a small back office medical aid, they have clients that are probably in the millions. You make mistakes and expect clients to pay for them, that is *******. Your mistake your responsibility.