Charged for Kulula's online error

I did online check in for a flight 5 min after check in opened. I chose a seat number and at the end of the process it told me i was checked in. Got to the airport 45 min before my flight, i was told that i had not checked in online and the flight was full. I was told to pay R490 for them to put me on the next flight. i desperately needed to get to DBN so i paid. My complaint is, why am i being charged for an error that was clearly Kululas? a few days before that when i tried to pay for a flight, the money was taken from my account but the booking was not done. i had to phone kulula to get them the manually fix this error. It is clear that their website does have errors from time to time! I want my money back for that flight change as it was not my fault that there was an error on the system! i did my online check-in and their system clearly had an error! It is also not my fault that they overbook their flights!!Not only did i loose money paying extra, i missed my business meeting!