Shouldn't need to purchase physical stamps right?
If its a parcel, you pay for it at the Service Point when sending (don't be confused, it's a shop).
If its a letter, you should be able to buy a stamp code online (https://www.postnl.nl/en/sending/letter-or-card/)

If it is indeed only a letter, there's also a mailbox landside, iirc between arrivals 3 and the car rental offices (so not the alley next to burger king but one further)