Aside from the issue that SR has been running for almost 3 years now without anyone introducing such legislation - they already scan the outsides of all letters or packages indefinitely, and can easily search anytime they want to. The legal niceties aren't why drugs by mail still work, it's because the USPS alone handles literally billions of things a year and the screening problem (economical, not too high false positive, USPS does not benefit in any way) is really hard.
*The people elected to run it.
“In the past, mail covers were used when you had a reason to suspect someone of a crime,” said Mark D. Rasch, who started a computer crimes unit in the fraud section of the criminal division of the Justice Department and worked on several fraud cases using mail covers. “Now it seems to be, ‘Let’s record everyone’s mail so in the future we might go back and see who you were communicating with.’ Essentially you’ve added mail covers on millions of Americans.”
Perhaps there could be a market on SR for little pieces of x-ray sensitive photographic film. Customers could pay providers a slight premium to include the film in the shipment. If the customer discovers that the film was exposed, they know that they've fallen onto somebodies radar.