It has long been held by US courts that the exteriors of letters and other items sent through the mail are not considered private.
It makes sense that they are allowed to photograph and record them for later use.
I mean, did you really think that a piece of mail sent through a government controlled organization would be hidden from law enforcement?
Now, if they are doing the same for UPS/FedEx/etc, then there might be a slightly larger concern, but still not really a big deal.
Or, if they were opening (or scanning the inside without opening) and recording the contents of sealed mail without a warrant, that would also be concerning.