If an item goes out of stock after a customer adds it to their cart, the site should handle it smoothly and honestly. Show a clear message in the cart and send an email or push alert. Offer options like waitlist or back-in-stock notice, substitute items with similar features, or remove the item and refund any payment. Keep checkout flexible: allow completing the order minus the missing item, or pause the order until the product returns. Provide an estimated restock date when possible, and fast refunds if they prefer. Make customer-service help easy to reach for questions.