Your customers have money, and you’d like them to give it to you. Payment gateways, such as Stripe, Braintree, and Paypal, make it easy to start charging credit cards and get the money flowing. But charging cards is only the beginning. You need to worry that your app responds gracefully to service failures, since charging a customer for a failed transaction is bad. You need to guard against fraud and security breaches. You need administrative tools that are flexible but secure. You want to test against external services. And you’ll run up against the law. Learn from some of my mistakes and build a robust financial application.