Booking fee should not change if they add feeders. It is always the same. 50EUR per pax plus 45 EUR (400 DKK/NOK/SEK) per person fi you book by phone. But the phone fee is waived when you have to call in, e.g. to book itineraries which cannot be booked online, like open-jaws.

Adding feeders in Europe usually works well, especially if booked on SK metal. I have no experience with the "other end", e.g. HND-ICN.