USPS has something called a "Mail Imaging program", where they photograph the outside of every piece of mail that goes through their system. They use it to process the mail, but law enforcement does request copies of all stored images related to people and addresses they're investigating.

They also have the "Mail Covers program", where law enforcement will proactively request USPS to monitor and forward information on all mail to and, as far as possible, from a given person or address. That information is limited to the outside of the envelope but, if they see something they deem suspicious, they can get a warrant to open the mail. I'll also note that privacy guardrails on the program are notoriously lax: something like 20% of covers were approved without the necessary paperwork, and 15% weren't adequately justified.