domain-ecommerce:cart-checkout

Cart and Checkout

When to use

• Designing or rebuilding a shopping cart (storage strategy, data model, merge logic)
• Implementing express checkout methods (Apple Pay, Google Pay, Shop Pay, Amazon Pay)
• Building or integrating a checkout flow (single-page, multi-step, guest checkout)
• Wiring up shipping aggregators or tax engines (Avalara, TaxJar, ShipStation, EasyPost)
• Building a promotion/discount engine with rules, stacking, and abuse prevention
• Setting up abandoned cart recovery sequences (Klaviyo, Omnisend, or custom)
• Implementing BOPIS with real-time store inventory and pickup notifications

Core principles

1. Validate the cart on every read — prices, stock, and promotions change; never trust a stale cart total
2. Express checkout above the fold — Apple Pay + Google Pay on product pages and cart converts before standard checkout even loads
3. Guest checkout is not optional — forced registration causes 10-25% abandonment; post-purchase signup instead
4. Shipping rate circuit breaker — always fall back to flat-rate if carrier APIs are down; never block checkout
5. Recovery sequence timing matters — T+1h (reminder), T+24h (urgency), T+72h (discount); discount-first burns margin

Reference Files

• references/cart-architecture.md — storage strategies (session/DB/Redis/headless/hybrid), data model, cart merging, cart validation
• references/express-checkout.md — Apple Pay, Google Pay, Shop Pay, Amazon Pay integration patterns and best practices
• references/checkout-flow-tax-shipping.md — single-page vs. multi-step checkout, guest flow, address validation, shipping aggregators, tax engine integration
• references/promotions-recovery-bopis.md — promotion engine architecture, discount types, Klaviyo/Omnisend cart recovery, BOPIS flow and technical requirements