domain-ecommercecart-checkout

Shopping cart and checkout flow design: cart architectures (session, DB, Redis, headless cart APIs), express checkout (Apple Pay, Google Pay, Shop Pay, Amazon Pay), shipping integration (ShipStation, Shippo, EasyPost), tax engines (Avalara, TaxJar), promotion/discount engine, cart recovery (Klaviyo, Omnisend), BOPIS (buy online pick up in-store), guest checkout, address validation, and checkout conversion optimization.