In practice USPS carriers everywhere I've lived completely ignore the check and drop it straight in the box. Good luck getting the government to prosecute themselves, particularly when ATF needs USPS for investigations against private individuals. And AFAIK, since the carrier requested to check the ID has no idea what's actually in the package, there's no mens rea to even prosecute them.

It's a legal loophole where the seller requests the check but the person delivering it has no binding liability to do so and they simply will not because it takes extra time. The economics practically guarantee the check won't be performed and the interface mechanics of carrier-seller means there's no practical way to prosecute either party when the carrier doesn't perform a requested ID check.