disallow transition "Exception" -> "Delivered" (this transition allows skipping of "In transit" and therefore the input of the shipping number)
transition "Exception" -> "In transit" requires to enter the shipping number again (ok if "Shipped" -> "Exception" but not if "In transit" -> "Exception")
requiring shipping number when re-entering "In transit" is ok (e.g. shipper is able to find lost seed unit and sends it as extra package -> new shipping number necessary)