Here is what the website says: "The World's Smallest Postal Service (WSPS) is a teeny tiny transcription service and roaming post office based in the San Francisco Bay Area and also available online."
"Your letter will be transcribed onto tiny stationary (1 x 1.5 inches, see example above). The tiny envelope even has a miniscule wax seal with your initial on it. The letter comes with a magnifying glass and the little letter package is put in a sturdy mailer for shipping. I can send your letter directly to a recipient of your choice, or to any other address."
Apparently if you order online your message isn't written by hand but is in type (but smaller).
You can also send a "Cute as a Button" package! The website says it's a "teeny tiny package with custom "to" and "from" addresses on it. Inside the little box is a strip of paper that says "You are as cute as a..." Under the teeny tiny crumpled newspaper, your recipient will find a sweet vintage button."
I think these would make a simple "thank you" or "happy birthday" super special. I'm not the only one who loves things in miniature am I?!