Businesses shall submit a transit declaration to Customs for undeclared goods that are transported within the EU and in Common Transit Convention countries. A transit declaration must be submitted also for undeclared goods transported through Finland.
You can submit a transit declaration to Customs online either as a message declaration or via the Transit Declaration Service on Customs’ website. ...
Submitting a transit declaration online via the Customs Clearance Service
The service is free of charge. You can find more detailed instructions in the service.