Not that I don't take your point that such a service could exist, but the site you linked explicitly says they don't offer letter writing as a service.
Also, I imagine it's not impossible to reliably distinguish between an autopen and genuine handwriting. The company who's site you linked say their machine can't perform complex pen movements so calligraphy is impossible.
The real advantage of posting a letter is that you have to pay for postage, and the stamps on the envelope will indicate which country the letter is really coming from.
If someone buys or makes something like this just to send a letter, they a) put in more human effort than writing it themselves, and b) are probably interesting and would benefit your community.
I think what robostwantdata was suggesting was that people would supply a "Use my mechanical pen for a fee" type service, where customers send the text they want written and the address they want the letter sent to.
As I've already said in my direct reply, there's still the various costs involved in posting the letter, and if the community is based on a particular geographical area e.g. "Connacht Hill Farmer's Association", then it would raise eyebrows if the envelope had a Kazakhstani stamp on it.
Also, I imagine it's not impossible to reliably distinguish between an autopen and genuine handwriting. The company who's site you linked say their machine can't perform complex pen movements so calligraphy is impossible.
The real advantage of posting a letter is that you have to pay for postage, and the stamps on the envelope will indicate which country the letter is really coming from.