Add seat and bag ancillaries to a Duffel order and handle order changes

Steps

1. After creating an order, retrieve available services by calling GET /air/orders/{order_id} and inspecting available_services[], or by having passed return_available_services=true on the offer request before ordering.
2. Each service has a type ('seat' or 'baggage'), id, total_amount, and for seats a designator (e.g. '14C') and disclosures[]; select the desired service ids.
3. Add services: POST /air/orders/{order_id}/services with data.add[]: [{id: service_id, quantity: 1}] and data.payment specifying the payment method; confirm the updated order object includes the service in services[].
4. To change passenger details (e.g. correct a name before ticketing), POST /air/orders/{order_id}/order_change_requests with the desired passenger updates; Duffel will return a change offer with a penalty amount.
5. Confirm a change by POSTing to /air/order_change_offers/{id}/actions/confirm with payment; the order will reflect the updated details.
6. For itinerary changes (e.g. adding a return leg), check if the airline supports order changes via Duffel — not all carriers do; if not supported, you must cancel and rebook.