I just begin to do part II and would like to get people's opinions about the relationship between Itinerary and Segment.
Since Itinerary has 1...Segments and a Segment can be an Itinerary (in this case Itinerary has exactly 1 Segment), Itinerary can be viewd as a composite and Segment as a leaf. When a customer would like to change the existing Itinerary, Customer selects the segment to change. In order to change, the segment is deleted and the Prepare Itinerary Use Case is executed. Using composite relationship, the pricing would be easy to calculate.