Personally, I would classify the US Post Office as a regulated vendor, and as such, not be involved in a regular review.
There isn't a whole lot of NPI, most likely. Two basic choices – exclude completely, or create a category that includes them, but doesn't require any recurring diligence activity, so it shows that you recognize the Vendor, but that you won't/can't pursue the more standard tasks, because of its federal status [ or whatever fits your internal policy ].
Thanks,
Dave
David Howe
Chief Information Officer