Imagine you’re on a beach in another country. You took a trip here on a whim and breezed past the currency exchange booth in the airport, figuring you could use your credit or debit card while visiting—no need for cash. In your rush, you forgot to bring sunglasses. A vendor walking along the beach has a pair that happen to be your style. In fact, they’re better than the pairs you remember passing in the duty-free area of the airport. Alas, he doesn’t have a credit card reader—just his Android phone—and you don’t have any local currency. He gives you a little card with an e-mail address and a phone number, in case you’d like to buy the glasses later.