Sick but unable to get a refund or change booking

I booked a Kulula flight in January 2015 to fly to Cape Town in April 2015. In October last year I was diagnosed with Graves Disease. In February this year the Specialist placed me on the highest dosage of the required medication and stated that by Mid April my hormone levels would be low enough to take the radioactive medication. Following this, I will need to be in isolation for two weeks. This means that I won't be able to fly to Cape Town. <br> When I phoned Travelstart, I was offered a R200 refund or the opportunity to pay R900 to change the flight. These are apparently Kulula's rules. I did present a letter of motivation from the doctor.