Payment Gateway API
Payment gateways in WooCommerce are class based and can be added through traditional plugins. This guide provides an intro to gateway development.
Payment gateway plugin base
This code can be used as a base to create your own simple custom payment gateway for WooCommerce. If not used in a custom plugin, you need to add this code to your child theme's functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Please don't add custom code directly to your parent theme's functions.php file as this will be wiped entirely when you update the theme.
Payment Token API
WooCommerce 2.6 introduced an API for storing and managing payment tokens for gateways. Users can also manage these tokens from their account settings and choose from saved payment tokens on checkout.
UX guidelines
UX guidelines for payments in WooCommerce.