What's wrong with stamp it?

What's wrong with stamp it?

I vaguely remember seeing something along the lines of "please affix correct postage" in the little square where the stamp is supposed to go.

Not a commonly used word, but means exactly "frankieren", also "freimachen". Mainly used is a context such as "frankly speaking..." (offen /frei heraus gesagt...)

... is what a BE colleague suggested when I asked for the translation a while ago

Hatte den Kontext zuerst nicht verstanden. Stimme opalo 100% zu!!!

Habe hier einen amerikanischen Rücksendeumschlag vor mir liegen, rechts oben ist ein Kästchen, in dem steht:

PLACE STAMP HERE
The Post Office will not deliver mail without postage

please affix correct postage

agree with Aika and BBee

BBee
das steht alles in dem kleinen kästchen ???

I recently received a letter from the ADAC - a 'Gewinnspiel'. The little stamp box has

0.55 € die für Sie lohnen!*

and here on the front of the envelope

*Nur ausreichend frankierte Einsendungedn nehmen an der Verlosung teil.

Also quite a bit for an envelope.

1)The "product" of Mariana's example is "a correctly/properly stamped, self-addressed envelope/post card"

2)"Freimachen (with proper stamp)" alone is "to stamp"
3) "Frankieren (with a machine)" is "to frank"

As to the wording maybe to be placed in a marked section on the envelope I like Aika's suggestion best