At the charming Hudson train station in New York, volunteers sell small cups of coffee for $1. The idea is to raise money for the children’s charity in town. They only accept cash. Not everyone carries cash. “Do you ever just let people have coffee on the promise that they will pay you later?” I ask. “Yes, and most do, but some people do not,” says the cashier. “We figure out who is who. When someone is taking advantage of our generosity, we say, ‘You may not have a cup of coffee. Next time bring cash.’ They figure it out...