Originally Posted by
Courmisch
If are not leaving or entering the EU, so you do not have to go through customs either way.
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Technically, at some airports at least, you are passing through customs even on intra EU routes, but you can choose the lane "Intra EU travellers". IIRC HEL has the same arrangement on exit?
For transfers at HEL you are supposed to report yourself at Gate 29:
If you are carrying goods that need customs clearance, declare them at the Customs service point opposite to Gate 29. If you arrive from outside the EU via another EU country, the declaration is submitted at the telephone points near Gates 16 and 21.
Passengers arriving from Norway and Iceland may declare customs clearance hand baggage and cargo hold goods at the telephone points of the passenger bridges.