X-Raying import of goods is one thing. What I'm thinking about is more opening letters to read writings and check pictures, without explicit permission nor judge involved.
This is exactly what parent is describing. If you’re on a suspicious person list, or happen to cross paths with one, your mail is likely going to be scanned. I can’t find the source now, but also once read the NSA can intercept a package, modify the contents and send it’s on its way without even a delay in the tracking.