Express One‑Click Checkout for WooCommerce (WooPayments PRB + Autofill)

Build a single-file WordPress plugin that enhances WooCommerce checkout with an express one-click purchase flow and shipping autofill, while leveraging existing payment gateways for Apple Pay / Google Pay.

Context: Site has WooCommerce and WooPayments active. WooPayments provides Apple Pay/Google Pay via Payment Request Buttons (PRB). We should not implement payment processing ourselves; instead integrate with WooPayments PRB where available.

Core features:

1. Express / One-click purchase button on single product pages:

• Add a button labeled "Buy Now" near the add-to-cart form (hook into woocommerce_after_add_to_cart_button).
• On click, perform an add-to-cart for the currently selected variation (if variable product) and then redirect to checkout with a URL parameter like ?p0_express=1.
• Use a small inline JS (enqueued) to submit the add-to-cart form via normal WooCommerce mechanism, but then force redirect to checkout. The simplest robust approach:
  • Add hidden input in the product form when express mode is requested.
  • Add a separate submit button with name="p0_express_buy_now" so server can detect.
  • On submit, let WooCommerce handle add-to-cart. Use filter/redirect after add to cart to send user to checkout if this express submit was used.
• Implement: use filter 'woocommerce_add_to_cart_redirect' to redirect to wc_get_checkout_url() with a query var preserving express flag.
• Ensure it works for simple and variable products. For variable products, rely on standard variation selection; submission posts variation_id etc.
• Only show for purchasable, in stock products.

2. Apple Pay / Google Pay integration:

• Since WooPayments is active, ensure Payment Request Buttons appear prominently.
• Provide admin setting to optionally reposition WooPayments PRB (if possible) or just encourage enabling.
• Implement a compatibility layer:
  • Detect if WooPayments gateway class exists (WooPayments) and PRB hooks exist; if present, add a notice-free integration.
  • Provide a product-page hook that prints a small wrapper container before our Buy Now button where PRB can render (some gateways hook automatically). To avoid breaking, we should not call WooPayments internal methods.
• Add a settings page under WooCommerce > Settings > Payments (or a small submenu under WooCommerce) titled "Express Checkout" with:
  • Enable/disable Buy Now button.
  • Button label.
  • Option "Show helper notice if WooPayments PRB not enabled".
• Frontend: if WooPayments is not active, optionally show an admin-only notice in wp-admin about needing WooPayments for Apple Pay/Google Pay.

3. Auto-filled shipping details:

• On checkout page load, if user is logged in and has a shipping address saved in user meta, prefill checkout fields.
• Use 'woocommerce_checkout_get_value' filter to supply defaults for shipping_* and billing_* fields when empty.
• Also support WooCommerce customer session values.
• Include a setting "Autofill shipping for logged-in customers".
• Respect user-entered values: only autofill when the field is empty.

4. Express checkout optimization:

• If express flag present (?p0_express=1), optionally hide cart page redirection and encourage direct checkout.
• Add filter to skip cart page if any plugin tries to redirect.

5. Security and best practices:

• Use nonce on express submit? Since it’s a normal add-to-cart post, WooCommerce already uses nonce for some forms; still add our own nonce field and verify on redirect logic.
• Sanitize and escape.
• Use options stored in wp_options with defaults.
• Provide i18n-ready strings.

6. UX:

• Minimal CSS for button styling matching WooCommerce button classes.
• Ensure button uses class 'button alt' and is accessible.

7. Logging:

• Add optional debug logging with p0_debug when redirecting in express flow (only if WP_DEBUG true or a plugin option 'debug'). But per requirements, p0_debug can be used without checks; still guard with option.

Deliverable: single PHP plugin file with:

• Plugin header.
• Constants.
• Activation hook setting defaults.
• Admin settings page.
• Frontend hooks for button, redirect, checkout autofill.
• Small enqueued JS/CSS.

Do not include any external dependencies.