There’s no regulation on content of the post, so you can encrypt your message, print it and send it by post. Equivalent of the court order in digital world is the permission to obtain whatever version of the content is available.

Mandating that all mail should be written in such a way that someone from the government could understand it, is clear overreach.

I agree with the argument on the logical level, but in practice I don't think it should be used be used, at least not as the first argument. For the general public, talking about encrypting a physical letter makes you look even more paranoid / malicious than when talking about online encryption.