If you are in a non Euro country you have to use the Finnair contact center. They have confirmed to me that they charge more for this service but there is no transparency on pricing. I recently booked flights using my voucher but found a big price discrepany - ticket with bag online - £290. Flight with bag on only one leg 400€ (about £360). I now have to buy the bag (something unclear when you book). All in all, it would have been better to take cash so my advice is never take Finnair vouchers.