When notifying the customer on the failed payment, Stunning gets the job done in three ways: In-app - Notifying your customer via app allows them to respond with valid credit card details in real time. Email - Stunning's Smart Retries feature lets you automatically and intelligently attempt to collect payments, and retry failed payments within 21 days. Your emails should still inform customers on failed payments, and what you're doing to resolve them. SMS - Sending text messages to your customers is faster, and is more likely to be opened and responded to right away.