Ticket taxes MUST always be paid at the time of ticketing. If they are not your employing airline (which is generally the ticketing airline) is liable for payment of uncollected taxes at the time of settlement. This means that if the taxes weren't collected at ticketing, and even if the check-in agent caught the error and collected at them from the passenger at check-in, the airline that lifted the ticket has the right to bill for them from the ticketing airline. Also, every ZED ticket requires an origin and destination, this is an agreed standard . An airline cannot issue a ZED ticket for a mileage band and expect that another airline will lift the coupon without question.
Also, the ZED agreement does not state that the traveler can expect to change his origin or destination, only that an airline may, at its discretion, allow that. Some carriers will not accept that at all because of the additional cost and complications it places on their checkin staff. Other airlines will allow it under certain conditions (e.g., same or greater fare is collected and the taxes do not change).
ZED tickets ARE route specific. As mentioned above, though, some airlines will accept a ticket on a different route, under specific conditions. Some simply have more flexibility in doing so. For example, a US carrier may allow different route/same zone on a purely domestic market because the taxes are the same. A UK carrier may not because the UB varies from departure airport to departure airport.
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