You must show a Clearpay checkout button on your cart and/or product pages to enable Express Checkout. When clicked, the Clearpay checkout button will start the checkout process in a popup flow (the redirect flow can optionally be used with Deferred Shipping).
Episode 49 - Express Checkout in Your Shopify Store - JadePuma
Payment Button Not Showing in the Shopping Cart - Community resources - Forum
WCS Hub - Tips and Tricks for Websphere Commerce: Paypal Integration with WCS 6.0
WCS Hub - Tips and Tricks for Websphere Commerce: Paypal Integration with WCS 6.0
PayPal - Intershop Communications AG
How to hide / remove Express Checkout buttons in Shopify
WooCommerce Buy Now Settings - StoreApps
API Simulator
WCS Hub - Tips and Tricks for Websphere Commerce: April 2012
WCS Hub - Tips and Tricks for Websphere Commerce: 2012
Are there any plans to implement Apple Pay for the createCheckoutButtonInitializer? · Issue #1028 · bigcommerce/checkout-sdk-js · GitHub
How To Set Up PayPal Express Checkout on Shopify
paypal-express-checkout-smart-payment-buttons - Event Smart
Express button