Apparently Vincent Van Gogh would often add small sketches and paintings to his letters to friends. BibliOdyssey has posted a collection of some of these wonderful pieces, along with the letters that accompanied them.
Four people on a bench
To Theo from The Hague
September 1882 
“Well, I hope that the small bench, even if not yet saleable, will show you that I have nothing against tackling subjects with something agreeable or pleasant about them, which are thus more likely to find buyers than things with a more sombre sentiment. [..]
There’s so much paint around that it has even got onto this letter — I’m working on the big watercolour of the bench. I hope it comes off, but the great problem is to retain detail with deep tone, and clarity is extremely difficult.
Adieu again, a handshake in thought, and believe me, Ever yours, Vincent”