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.
But do I really need to explain why that is undesirable, or why most people in the world don't do this?
Maybe they have to tweet you the advertisement, and then you can retweet, so you get more exposure in their twitter stream as well.
Honestly, why? You can't scold everyone into "real" virtuous behavior, so why not give them a taste of what it feels like and see if they can get hooked?
Do you believe that people putting money on this card would actually donate it towards a more deserving end?
I (think I) understand your goal here, but ultimately I think what you're doing is either ineffective in or outright harmful towards achieving it.