So it has come to my attention that at least some countries use automated mail sorters that are capable of deciphering human written addresses. More importantly, some countries are starting to store databases of all mail routing information. I was told this by a very trusted source, although I am digging around for technical specs of mail sorters right now (love open source intelligence!). This is scary.
Let's say Alice buys a pack from Bob. Bob is known to ship from netherlands. Alice gets the pack with no interception. WOOT. Then Alice leaves feedback on Bob saying that she got her package. Next Alice orders from Carol. Carol ships from New York. Alice gets her package with no interception, and then leaves feedback on Carol.
The DEA is not happy. They want Alice to fuck off. So they query mail sorting records and look for everyone who got a package from Netherlands in X time frame around when Alice left a review for Bob, in addition to everyone who got a package from New York around the time frame that Alice left a review on Carol. Lots of people are probably in both of these crowds. Now they intersect these two crowds, and out pops Alices shipping address.
My source tells me that he is absolutely certain and knows for a fact that some nations have sorters that log this information. He is not sure about other countries.
edit: The person who told me this says it is unlikely that police agencies can (legally) gain access to this information, but that intelligence agencies certainly have access to it.
https://en.wikipedia.org/wiki/Optical_character_reader
http://www.toshiba.co.jp/sis/en/scd/postal/lsm.htm
edit II: hmm maybe police can legally gain access to this information....
https://ssd.eff.org/book/export/html/16
Postal mail. The mail that you send through the U.S. Postal Service is protected by the Fourth Amendment, and police have to get a warrant to open it in most cases.
If you’re using the U.S. Postal Service, send your package using First Class mail or above. Postal inspectors don’t need a search warrant to open discount (media) rate mail because it isn’t supposed to be used for personal correspondence.
Keep in mind that although you have privacy in the contents of your mail and packages, you don’t have any privacy in the "to" and "from" addresses printed on them. That means the police can ask the post office to report the name and address of every person you send mail to or receive mail from — this is called a "mail cover" — without getting a warrant. Mail covers are a low-tech form of "traffic analysis," which we’ll discuss in the section dealing with electronic surveillance.
You don’t have any privacy in what you write on a postcard, either. By not putting your correspondence in an envelope, you’ve knowingly exposed it, and the government can read it without a warrant.
www.cryptome.org/isp-spy/usps-spy.pdf
USPS mail cover procedure
edit III: In some countries only intelligence agencies can legally access mail routing information without a specific warrant (law enforcement can check routing information of certain people with a warrant, but they can not do blanket analysis of stored routing information), USA is *not* one of those countries (USA citizens have a right to privacy of mail contents, but not routing information....there is no law preventing police agencies from doing the attack mentioned in this post.)
edit V: Apparently it is against USPI procedures to use mail covers as an initial investigatory technique. I am not yet sure if this is protected by law or not, but from the EFF information it appears not to be.
Mail covers are issued only to agencies empowered by statute or regulation to conduct criminal
investigations and are strictly controlled to assure proper use. They are not to be used as an initial
investigative step.
1. Mail covers are not authorized for exploratory purposes or for crimes punishable by
less than one year imprisonment (misdemeanors).