Time to stand up against doctors who add to your bill

This is my name and shame experience at Dr Terblanche dentist in White river. We made the appointment with specific instruction to only do what the medical aid and our specific plan would cover. No other procedures will be paid. Opened a file as per normal for the first visit. They approached me stating they need permission to take an x-ray not covered and what the cost was, I approved that procedure only. All done happy and we left with no other word of extra monies to be paid. Now they phone for outstanding monies and are adamant they will receive payment. After patiently trying to explain that I did not Authorize the extra work the only comment is, you signed papers that you will pay (opening the file). What gives them the right to just add onto your bill whatever they want after direct instruction to only do what the medical aid will pay. Totally pathetic and unethical way to earn money by adding on services and then just bill without authorization.