Every time people discuss monetizing open-source projects, I tell them about #Blender. They announce a feature to be added or a movie to be developed, ask you to buy it, and you get the feature or movie. I bought several of their movies, books and paid for features.
Other project sadly don't go beyond a donation button. You pay a donation and you get nothing tangible in return.
I remember one time Blender offered sightly overpriced blender-themed USB sticks. That seems like something any open-source project can do.
Just offer something in return and people will pay for it. It actually feels very nice when a feature comes to fruition after I paid for it. It's like I paid for something that benefits the world. A guy in Africa who wants to try creating 3d art will benefit from a specific feature I paid for, and some developers are nice enough to immortalize your contribution by adding your name to the list of "donors."
Example: