David,
Actually I see the confusion you have. Its a valid confusion when you analyze the use cases and the use case model. However I think we need to make certain logical assumptions based on our experience.
This is what I have assumed.
1. Prepare Itinerary does not include Pay for Itinerary and therefore the Post Condition of Prepare Itinerary now becomes valid with this assumption.
2. Since Prepare Itinerary does not include Pay for Itinerary, the use case model become valid when a direct connection is shown from the customer to the Pay for Itinerary use case since the customer pays for itinerary once it is confirmed which is what I have assumed as the outcome of Prepare Itinerary.
With few such assumptions, everything starts falling in place
.
Regards
Raj