Post-purchase upsell with BlockStack on the thank-you page added 11% to AOV in 3 weeks of testing Shipping-step gift-note collector replaced a 19 EUR/month app with 90 lines of preact and zero merchant cost Loyalty discount line at cart-line-item.render-after pulled live points balance from a custom API and wrote a CartLineDiscount via mutation Free-shipping bar at delivery-address.render-after switches threshold per country with a single useShippingAddress hook I shipped four Shopify Checkout Extensibility UI extensions in April. Three on Plus stores, one on a standard plan. Same React-flavored preact, same target strings, very different surface areas. If you have not migrated off checkout.liquid yet, the deadline is real and the new model is honestly easier once you stop fighting it. The catch is the targets. There are dozens of them, the names are long, and picking the wrong one is the difference between an extension that renders and one that quietly does nothing.…