I need to pay my Amsterdam municipal taxes. Instead of making one payment for the whole year, the municipality offers you this form to enable 8 monthly payments.
Now, the thing is, it's a one-page form with no attached envelope. It says on the bottom:
Stuur deze ingevulde machtiging naar de gemeente Amsterdam ... blah blah address here ... Een postzegel plakken is NIET nodig.
i.e. essentially "Send this completed form, postage stamp not required". Well, ok, but - do I just drop this piece of paper in one of the orange postal boxes? Also, what about my account details appearing just like that for everyone to see? On the other hand, if I do put it in an envelope - won't the postal service expect a stamp?