Vatfree.com
Where can I get a customs stamp? / To be eligible for Vat refund your receipt has to be stamped at the airport (or harbour) where you leave the EU.You can get a customs validation stamp at a customs office at the airport.
For The Netherlands we provide a map: Schiphol Airport Customs.
For information about where the customs office is located at your specific airport of departure, please contact the airport or ask an airport employee when you are checking in for your flight.
Please make sure to arrive at the airport on time. You will need some extra time to visit the customs office to get your stamp, and be on time for your flight.