Courier is a platform that makes it easy to send SMS, email, push and in-app notifications with a single API call, providing a REST API, client SDKs and documentation for developers. The author discusses the concept of toast messages, which are discreet but noticeable messages delivered to users for feedback or direction, often found in mobile apps, web browsers, video games, and other applications. Toast messages originated on Android devices but are now widely used across platforms. They are used to deliver feedback, confirm user actions, direct users to specific parts of the app, or introduce new features. The author notes that toast messages differ from other messaging formats like traditional persistent messages, push notifications, snackbar messages, and in-app notification centers. Courier's notification platform allows for easy implementation and integration of toast messages into a multi-channel control system, making it easier to choose the right SMS API for projects.