The OrderChange request allows to add/remove/update an element to an existing order. The upsell and display of the ancillaries and seats can be done: - Via the SeatAvailability in order to request and return a seat map for a selected segment and/or - The ServiceList verb that can be called to return the Catalogue of Ancillary Services The OrderChange verb, when called the post-sale booking flow will allow and orchestrate: - The booking of the seats and ancillary services - The payment processing - The issuance of the EMDs for these services