Original poster here reporting back.
One can post packages at the "Service Point" which is located in Schiphol Plaza opposite Hema (I did so a few days ago.) It is easy to miss, because most of the shop is devoted to shoe repair, but they can post parcels and give an official Netherlands Post receipt and tracking number. If you are facing the gates with the roadway to your back it is at the far left end of the shops.
Here is a link to a photo and description:
http://www.schiphol.nl/Reizigers/Nie...ipholPlaza.htm