Sometimes it's a voluntary donation, sometimes it is not and you'd better pay the koha.
Sometimes it is used as a blatant bribe to grease the wheels. (but koha sounds so much nicer than bribe doesn't it
Traditionally it's food, but not these days. Cold hard cash thankyouverymuch.
(True story - koha in action
There's a band that travels the North Island here and plays in the various small town pubs.
With each given pub they have a choice; voluntary door charge (koha) or, the pub pays them a flat fee up front.
This band always goes for the flat fee, with the exception of one pub not far from where I live.
When they play at that small town pub once a year, they always ask for koha at the door.
I asked the band members what was so special about that pub?
"It takes us six months to smoke the koha"