functions.php

<?php
/**
 * @snippet       WooCommerce - Fortnox plugin by Redlight Media - Only create invoices that has payment method Swish
 * @author        Redlight Media AB / Christopher Hedqvist
 * @compatible    WooCommerce 3.9.2
 */
function redlight_fortnox_create_invoice_for_swish_orders( $create_invoice, $order_id) {
    $order = new WC_Order($order_id);
    if("redlight_swish-ecommerce" === $order->get_payment_method() ){
        $create_invoice = true;
    }else{
      $create_invoice = false;
    }
    
    return $create_invoice;
}
add_filter('obj_fortnox_create_order_create_invoice', 'redlight_fortnox_create_invoice_for_swish_orders', 10, 2 );