I need multiple product cart capability added to our custom book builder. We’re using Shopify with a custom Node.js + React app, and currently customers are sent straight to checkout after creating a book. We need proper session management (via the Shopify Cart API) so customers can add multiple personalised books to the same cart before checking out. Each book is a unique customised product with its own configuration data, so the session must persist these across multiple builds without overwriting or clearing previous entries. The solution should ensure each custom book instance is stored correctly, session isolation is maintained, and cart data remains synced with Shopify checkout. Creating a Shopify app extension and moving the frontend into the extension for proper integration with Shopify’s Cart API, Updating the backend order flow, including book creation and PDF generation logic, so it works with multiple cart items. And adjusting our AWS queue (BullMQ) handling to manage multiple jobs concurrently.