Customs at Helsinki Airport have two locations...
Customs Export Clearance service point located at the arrivals hall
Gate 29 (if you are a transfer passenger) (Schengen area)
Head to the tax refund desk when you arrive at the airport for your return/transfer flight.
Do this before checking in your baggage (if not on a transfer) to make sure you have all items and receipts for customs inspection - it's an easy mistake to make but if the customs can't check your items they can't stamp your claim 💮
The Customs Desk on the airport grounds is open at all times however there can be queues for this service so we recommend that you arrive at the airport within plenty of time to be able to get the stamp.
Contact
Airport Customs – Helsinki-Vantaa – Air passengers
Postal address: PO Box 512, FI-00101 Helsinki
Every day around the clock