The Postal Service on Friday confirmed that it takes a photograph of every letter and package mailed in the United States — about 160 billion pieces last year — and occasionally provides the photos to law enforcement agencies that request them as part of criminal cases.  http://www.nytimes.com/2013/08/03/us/postal-service-confirms-photographing-all-us-mail.html?ref=us  The images are taken at more than 200 processing plants around the country. The Mail Isolation and Tracking system was created by the Postal Service after the anthrax attacks in late 2001 killed five people, including two postal workers.
Tens of thousands of pieces of mail each year undergo this scrutiny, and a number of law enforcement agencies have used it, like the Federal Bureau of Investigation, the Drug Enforcement Agency and the Department of Health and Human Services.
The digital mail tracking programs had raised concerns about their sweeping nature because the post office and law enforcement agencies are allowed to monitor all mail, not just the mail of those suspected of a crime.