I've not used postal facilities at Schiphol for some time, but I think you'll be able to post a parcel at the postal facilities at the Communication Centre on the top level above the Lounge 2 area. I can't find an obvious direct link, but clicking on the Communication Centre link at http://www.schiphol.nl/portal/faciliteiten.jsp? suggests parcel post is available there.

There is (or was? possibly this might have been replaced by the Communication Centre?) also a small post office near the G pier in the Lounge 3 area. This still appears on this Schiphol terminal map but I don't recall if it's still there. Hope this helps - perhaps someone else here may have more up-to-date experiences.

Edited to add that it looks like Kris Rao and Dambus do have more recent experiences, and just beat me to it. ^