Bad after-service

We never got on the airplane.

Air Namibia sold us the ticket AFTER the gates were closed for boarding. We missed the flight and I've been trying to get our money back since August 2019. They admitted the were wrong, which I've got in writing. They then payed just over half the amount due, now saying we need to pay penalties. If the mistake was with the airline, as admitted, why are the passengers paying for it? It's R2000.00 in penalties.

I will never fly with this airline again, as this is not our first bad experience with them. My parents have property in Namibia, so we fly there often. I will rather pay more and avoid this airline at all cost.