If you were living in Ancient Babylon (six thousand years ago) you might bake a letter along with your daily bread.
Messages were written on clay tablets, which were then baked to harden them. The tablets were then covered with more clay and baked a second time.
The inner tablet could only be read by breaking open the outer layer of clay. The 'secret' message was thus secure. A hammer is my kind of letter opener! That would truly be a 'vintage' postcard. Here is a tidbit from the Babylonian epic poem Gilgamesh. I wouldn't have minded having this sent to me.
"Why, O Gish, does thou run about?
The life that thou seekest, thou wilt not find.
When the gods created mankind,
Death they imposed on mankind;
Life they kept in their power.
Thou, O Gish, fill thy belly,
Day and night do thou rejoice,
Daily make a rejoicing!
Day and night a renewal of jollification!
Let thy clothes be clean,
Wash thy head and pour water over thee!
Care for the little one who takes hold of thy hand!
Let the wife rejoice in thy bosom!"
Here is the link to the full Gilgamesh poem which deals with the struggle
between cosmic order and primeval chaos. Now that's heavy!