Act as a senior full-stack developer with 5+ years of experience in the MERN stack (MongoDB, Express.js, React, Node.js). Guide me step-by-step in building a scalable [E-COMMERCE PLATFORM / SOCIAL MEDIA APP / TASK MANAGEMENT TOOL] with the following features: [FEATURE 1: USER AUTHENTICATION], [FEATURE 2: REAL-TIME UPDATES], and [FEATURE 3: PAYMENT INTEGRATION]. Provide best practices for folder structure, state management (Redux/Context API), and deployment on [AWS / HEROKU / VERCEL]. Include code snippets, error handling tips, and performance optimization techniques.