Interesting. From a strictly technical perspective (this is impractical), we could set up a warehouse somewhere in the country which accepted incoming mail (tagged with an ID number), dumped the entire package into a plain manilla envelope, then sent it on to the final address.

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.

You could send mail to the wrong address intentionally. Like you want to send mail to 3 Main Street, but you write "5 Main Street" instead. The person who receives it will say "Ah, this is for Joe. He lives next door!" and hand deliver it.

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.

By putting another letter inside that may or may not contain another letter ad infinitum, where each letter is sent to a random address except the most inner letter which is sent to the actual address. In each outer letter you put a five dollar bill and the words "Mail this or else, I know where you live!"