Could you freeze-dry your material before transport? You could use a vacuum desiccator or drying agent (silica gel). Once the material is dry, and packed in the dark in an airtight bag or container, it should be easy to transport (and much lighter).

IATA dangerous-goods rules allow you to check the cool-dry box on the plane, which increases your weight limit.

There is also a company, Cryoport, that works with the shipping companies to transport scientific samples in liquid nitrogen. An intermediary like this gives you more flexibility than trying to deal with FedEx et al yourself. They have an office in the Netherlands.

It may be cheaper to arrange to do the extractions at a Finnish university or field station.

I just wanted to update the result of this. What I ended up doing was ordering dry ice to my location which was easier and cheaper (and what I would have had to do anyways to ship it). Then I got a cooler box and took it on the plane as checked luggage. As long as the dry ice is under 2.5 kilos and labelled properly you can bring it onboard. Since the travel time was about 12 hours and I was able to get it to the university afterwards this was a good solution if anyone else needs this advice.