I briefly attended a few local Mycologist Club meetings and they mentioned that they pulled in more cash at their last annual Wild Mushroom show after going from a $2.00 admission to a "donate to the club" admission.
Several times when I've gone to a restaurant with a large group of people, we were hit with a 15% gratuity. None of us ever had a problem with it, but discussing it later, we concluded they would have made almost twice that if we had all tipped individually.
And I figured that #36 was a third-party confirmation that the "donate what you deem appropriate" scheme is a solid one.
Fish bones FTW!!