Fair point. My goal is to figure out the dynamics of social giving in this fashion and create a similar sustainable model around items that are less "luxury" than coffee.
For example, I can imaging people paying for other people's groceries or medicine in a similar manner. Maybe I'm crazy, but it seems worth trying.
I hope one of your other ideas works! Good luck! But I suspect a big part of the impetus behind the contributions is a feeling of reciprocity, which is a different impulse from charity.