I sent off a letter in Braille to the States on Monday… I hope it'll get there all right.
Apparently, the United States Postal Service and the German Post Office have different criteria for which items are eligible for free posting, as I found out while searching for how the USPS would like to have such items labelled.
Part 274.2 of the USPS International Mail Manual says that eligible mail must be marked "FREE" and "MATTER FOR THE BLIND"; as for what items are eligible, part 270 refers to part E040 of the Domestic Mail Manual, specifically to E040.2.0, which says, among other things,
The matter must be for the use of a blind or other physically handicapped person. (check) but also that
Letters prepared in any form by sighted individuals […] may not be sent free and must bear the full applicable postage. Oops; bummer.
However, I figure that if the German post office accepts the item for free mailing, the USPS should carry it as well; the German policy is explained (in German) on this page and say that "anyone" can send certain items free of charge, including internationally, among which are
Schriftstücke in Blindenschrift (Braille-Schrift) (written documents in braille), with no mention of eligible senders or recipients.
Which is kind of cool; I can send letters free of charge throughout the world! Though obviously only to recipients who can read Braille, and only via surface mail.