How can I pay for extra baggage if one of the flights in my itinerary is not operated by Finnair?

You can pay extra bag charges in advance if all the flights in your ticket are marketed and operated by Finnair. If you are travelling on a flight operated by Finnair but your ticket has been issued by another airline, or your itinerary includes flights operated by another airline, please pay for your excess baggage at the airport. Please note that cash payments are not accepted for Finnair services at Helsinki Airport.