As I implied in my top level comment, it should happen more often than it likely does. If you work on a commercial project with any GPL code ask your test group who has done that and when - if you don't see a lot of hands go up then your test group isn't doing their job. (if you are only automated tests, then I assume you have an automated test to send this letter and verify the response)

For April fools I should set up an API for sending postal mail as a service