Some wanted to have a definite cover charge, some wanted to make it clear that any money given was to be considered a donation and anybody could come in whether they gave money or not. But for whatever reason, that idea did not go over. Propaganda stuff like pamphlets, tshirts, etc. A fellow I worked for one summer told me about his friend who had several tons of horse manure and wanted to give it away for compost, so he put up a notice at a local store to the effect that he would deliver it for free. It's often easier to get people to take something if they have to pay for it, even a tiny amount.
People are just often suspicious of what's free.
Look at old copies of revolutionary or subversive pamphlets from the last century. Many are marked Ten Cents or Five Cents, which was a lot more then than it is today. Hitting people up for nickels and dimes won't keep the doors open by itself but it might show people we take what we make seriously enough to ask some money for it. One of the best projects to ever come out of the Peace Center was a little zine called BloomingPeace. Bookstore This would do the business of a regular bookseller, buying any old books at low prices and marking them up.