I don't mean to sound nitpicking, but you're using the word "secure" but I think your method is really more about "privacy". They aren't the same thing although a method can be both secure and private. Are you looking more for privacy?

A PGP encrypted email can be highly secure and difficult to read the contents, but who it's from and where it's going may be plainly visible, so that aspect of the message is not private. So, NSA's PRISM program would dutifully record that kind of info.

The method you described seems quite private, but not necessarily highly secure unless you're using written codes or encrypting digital info that you're sending this way. It used to be that a search warrant was required to intercept and open first class mail, but I think that bar is significantly lower nowadays, especially in cases where national security is used as a justification.

Not to freak you out further, but here's a very recent article describing the Postal Service's version of PRISM.