Shopify Checkout Extensions Overview Shopify Checkout Extensions allow apps to render custom UI blocks inside Shopify's checkout without forking the checkout template. Shopify Functions let you replace backend logic — discounts, shipping, payment methods, and order validation — with custom WebAssembly modules that run inside Shopify's infrastructure. Together they replace the deprecated customization approach and work with both Shopify Plus and non-Plus stores (UI extensions) or Plus-only (some Functions targets). When to Use This Skill - When adding custom UI blocks to checkout (upsells, tru…