The Question: “WooCommerce only sends invoice when triggered in the Dashboard. How to send these invoice automatically?”. Stackexchange.com WordPress Development does not allow WooCommerce questions any more. That is why i post the answer for this question here.
Add an action to your woocommerce_order_status_completed_notification or woocommerce_order_status_pending_to_processing_notification in functions.php:
function sendinvoice($orderid)
{
$email = new WC_Email_Customer_Invoice();
$email->trigger($orderid);
}
add_action('woocommerce_order_status_completed_notification','sendinvoice');