I do so love snailmail (who doesn't?), and this idea is so clever... It would be so fun to get this in the mail!
The "postcarden" is actually a shallow box, and it comes with a packet of seeds. You water the seeds for two weeks, and you get a small crop of cress, which is also edible!
It's currently only available in the EU, but Aimee, the creator, plans to expand production soon.
The Postcarden by Aimée of A Studio for Design
via The Benevolent Postcard Society