Overbooking by Flysafair

I was booked on the 6:40 flight to East London. Got to the airport at 5:30. The boarding board said the flight will start boarding at 6:20. Got to the gate at 6:23 and was told the flight is closed as it closes 15 min before departure time and that the flight departs at 6:30. I explained to the staff that my ticket says my flight departs at 6:40 not 6:30 as they mentioned and that I was not late. They wanted to hear non of it and now I had to pay R300 to be on standby and I have to seat at the airport for 5 hrs waiting and hoping that I will get in the next flight.