I connect the plug-in to Invoice express by configuring my subdomain and my API key, then I generate a new (test) order from WooCommerce. Lastly, I promote the order to "Processing" step and save the order.
What's supposed to happen is the API gateway is supposed to duplicate the order details (client, billing/shipping addresses, products, prices, subtotal, VAT, etc.) to InvoiceExpress and automatically generate an invoice for the order.
What actually does happen? nothing.