Thanks a lot for your prompt reply.
I did that and it works. However, I need to rather have the order number from the database displayed on the order confirmation as well as in authorizeNet. We can not use the session ID as an order number since it is far to long and creates problems with our order processing.
Is there a way in the authorizeNet behavior in the confirm.php page to have the order number that is created by the database transferred to authorize net rather then the session number?
Could I for example change the value in the x_invoice_num field from <?php echo session_id(); ?> to <?php eCart1_OrderID(); ?> or something like this?