Changing my booking a Nightmare!!!!

I am a regular flyer at Kulula - flying with 4 passengers every 3 months. I had booked a flight in advance for the 13 of Feb to 15th Feb from JHB to DBN return. Unfortunately I contracted Pink Eye which is highly contagious so I did the responsible thing cancelled my holiday and called Kulula to change my flight. I provided a doctor's certificate to validate the illness. I was told that they could not accept the certificate as it did not have a stamp on it. As a result I was liable for R350 penalty change fee per passenger, per flight plus change in the flight prices. I paid an additional R2800 to have my flight changed. When asked if I could just keep my original flight as I thought the cost was excessive I was told sorry that flight has been sold (within 10 minutes) and is no longer available. Until I paid the additional R2800 (My original ticket was R4300), I would have no confirmed booking with Kulula. This sucks- u are ripping customers off and have no idea how to treat good customers. Here's hoping that your monopoly on the domestic market ends soon. You have left a bad taste in my mouth and will share my experience with all I know!