How to send a package internationally (to Finland), from the US?

The box is fine. Seal it with clear package tape. /Write the address exactly as your friend gave it to you. Write " Finland" in English. Take it to the post office. They will weigh it and tell you the stage and stamp the box with the correct amount. You will not purchase stamps for it. They will give you the customs paper to complete. There is o fee for that. If you are unsure about how to write the address, wait and ask at the post office...just have it on a piece of paper with you.