cachet

1. A seal, as of a letter.
2. A special characteristic or quality; prestige.

   I remember when this diner was a quiet hangout, but lately it seems to be losing its cachet.

3. A commemorative stamped design or inscription on an envelope, other than a cancellation or pre-printed postage.
4. A sealed envelope containing an item whose price is being negotiated.
5. A capsule containing a pharmaceutical preparation.
6. A hidden location from which one can observe birds while remaining unseen.

verb

To mark (an envelope) with a commemorative stamped design or inscription.