Design an E-commerce Checkout

Requirements

An e-commerce checkout turns a customer's cart into a paid order whose items are reserved in a warehouse. Browsing a catalog can be sloppy; checkout cannot.

Functional:

• Search and browse a catalog of products and sellers.
• Add items to a cart that survives across devices.
• Place an order: confirm prices, reserve stock, take payment.
• Show order status; cancel and refund before shipping.

Non-functional:

• Do not oversell (sell stock you do not have) beyond a small, deliberate margin.
• Never double charge, even if the user clicks twice or the network retries.
• Catalog and cart stay available during failures; checkout may fail safely.
• Survive flash sales where one item draws a huge crowd.