Environment
Standard WordPress/WooCommerce (WP/WC) deployment with custom payment plugin. On click "Place order" button, the checkout page is blocked awaiting customer interaction. Once received through external server callback the checkout process could continue or be canceled.
Goal
Enable WP/WC callback URL for receiving asynchronous responses from external server so the checkout page and process could be updated accordingly.
Request
WC_API – The WooCommerce API Callback seems like the way to go, however I am unable to get it working and need a step by step guidance or a pointer to a description on how to do that.