The Pay for Itinerary use case specification describes an alternative flow that is performed if the user does not satisfy a requirement.
Main flow: 1. User does something. 2. [constraint] system does something. 3. User does something.
If !constraint: 1. system does something new before the original something. (this copyright law makes it difficult to ask a simple question)
Later in the specification a pre-condition states that the user must satisfy this requirement. So why is the [constraint] there in the first place? The pre-condition states that the user satisfy this constraint.