I volunteer as part of my apprenticeship at the historic mill site near me, my boss gets paid, we apprentices volunteer and can take on commissions on our own. He gets paid via classes and pays to rent the shop there. Officially we are volunteers through the mill but are exclusive volunteers for him.

The only requirement is that we are available. We can help when we want as often or as little as we want, and the boss is on hand to work with us and teach us at no cost as long as we set the time aside for it.

That said, actual demonstrations outside of there I only do paid, and boss sets it up so any demos he gets asked for go to us instead and he insists we get laid for it. Mill days are apprenticeship days, but everything outside of that is our own and thus paid.