Full-Stack: Shipping Label Web App (Crypto Payments)

The Gist: I need a robust, bug-free web app that acts as a simplified version of ShipStation. Tech Stack: Node.js or Python Backend (NO PHP). React/Next.js Frontend. Payment: 100% USDT via Escrow. The Flow: User: Signs up (Email/Pass, NO email verification needed) \(\to\) Logs in. Wallet: User clicks "Add Funds" \(\to\) Redirects to Crypto Gateway (e.g., NowPayments/BTCPay) \(\to\) Pays USDT/LTC \(\to\) Redirects back \(\to\) Balance updates. Action: User fills out a shipping form \(\to\) Clicks "Buy Label." Backend: Checks Balance \(\to\) Deducts Cost \(\to\) Calls My Custom API \(\to\) Returns PDF Label. Key Features (Must be flawless): 1. The "Create Label" Form: Constraint: US-to-US addresses only. Address Book: Auto-save new addresses. Dropdown to auto-fill previously used addresses. Randomizer Button: A small button next to email/phone fields that, when clicked, auto-fills a random realistic email/phone number. Duplicate: A "Clone" button on past orders that pre-fills the form with that data for a new shipment. 2. The Dashboard: Simple table of history (Date, Recipient, Cost, Tracking #). "Download Label" button. 3. Settings: Simple page to update Password and Email. Technical Requirements: NO PHP. Preferred Stack: MERN (Mongo/Express/React/Node) Mobile Responsive: The "Create Label" form must work perfectly on mobile devices (no zooming issues, broken inputs, or scrolling bugs). Concurrency Handling: You must ensure that if a user has $10, they cannot fire two API calls simultaneously to spend that same $10 twice. What I Provide: The API: A custom REST API endpoint for label generation. The Design/Flow: Access to a live example site to mimic. The Gateway: API keys for the Crypto Payment Gateway.