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 know, you're not obliged to put a return address on regular mail as long as it has sufficient postage. It's required for various kinds of commercial mail, or mail requiring special handling eg delivery confirmation).
Apparently (https://news.ycombinator.com/item?id=5986011) the letter can still be tracked by looking up the return addresses and point of origin of letters with neighboring unique IDs.
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.