The only real solution is to have end-to-end encryption that is easy to understand and use.
The only real solution is to have end-to-end encryption that is easy to understand and use.
That is a great way to get your letter not mailed. A simple "Please" would work much better.
The final address would be set up via an encrypted web service, so only you and the warehouse know who sent the mail & where the final destination is.
Of course, then the NSA would just take the warehouse's private encryption keys, so it'd only work for about 48 hours, but yeah, you know. Technically possible.
That's mostly sarcasm. There's no getting around the tracking if you want to use regular USPS mail or Google Gmail. Just go meet in person, and leave your cell phones at home.
[1]There's still the problems of metadata and backdoors and of legislation around encryption.