A shipment notification service is built using Python, Flask, Twilio, and EasyPost APIs. The service allows for automatic tracking of shipments and sends notifications via SMS when a shipment reaches certain stages, such as being out for delivery or delivered. The service handles webhook events from EasyPost, processes them asynchronously, and uses Twilio to send SMS notifications. It also includes features like event deduplication, security measures, and the ability to scale horizontally. The service is designed to be scalable and can handle high volumes of traffic.