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Where can I buy envelopes and postal stamps at Schiphol? /

Postal Services at Schiphol airport, Amsterdam

You can buy envelopes and postal stamps in the shop 'Service Point Schiphol Plaza'.
Find 'Service Point Schiphol Plaza' on the map here.
Service Point is located at the arrivals level, between arrivals 3 and 4, next to the shop 'Rituals'. 
Open 7.00-22.00 daily.

Mail Drops (orange postal boxes)