Discovery not paying at cost as arranged

I changed my medical aid some years ago to pay at cost. Meaning that whatever claim I put in they will pay the full amount and there will be no co-payment. This means my savings gets depleted faster which I am prepared for.

However for my Rheumatologist they have a special payment arrangement which simply overrides my pay at cost arrangement. Although I can easily pay the co-amount I am highly ****ed off as it is the principle of the matter.