"The USPS uses a program called Mail Imaging, which takes a photograph of the outside of every piece of mail sent in the U.S. While the primary purpose of taking the pictures is to process the mail, law enforcement agencies are able to request images of mail sent and received by a target they are investigating."

https://www.newsweek.com/postal-service-photographs-every-piece-mail-us-shares-agencies-request-it-280614