Federal law prohibits either the sender or the carrier from marking the package as containing a firearm. Therefore when it comes to following the letter of the law, the law itself makes the carrier's job somewhat difficult.
Yes it does, that's why the wording states 'knowingly deliver'. The driver has no way of knowing what in the package, therefore he's not in violation if he inadvertently delivers a firearm without a signature.
However, you and receipient are certainly AWARE the that package you are due to receive contains a firearm, unlike the driver. So if you want to make sure that federal laws are followed (and who doesn't?) then either be at home and be ready to receive the package containing firearms or post notices asking your packages to he held if you are not home to sign for them.
Ironically, requesting 'hold at depot' is what I do with almost all deliveries of anything because I work 2nd shift and UPS has the bad habit on my route of making home deliveries after I have to leave for work.
Sometimes that doesn't work either, though. My M38 Nagant from Aztec had 3 UPS 'hold at depot' labels on it with my phone number.
Imagine my surprise when I got home from work 2 days later and found the box had been delivered. My landlord signed for it, but that wasn't what they were supposed to do.
My point is that the best thing to do is just follow UPS's rules
in re shipping firearms. Don't lie about what's in the package, make sure that it's sent 'adult signature required', hold on to your UPS paperwork, and then relax. You've done your part.
If UPS screws up and delivers it to the wrong address, if it gets stolen by someone working for UPS, or if it's left out on the porch and your neighbor's 16 year old steals it, you're legally covered, the gun is insured, and UPS is in trouble.
If you've lied to them about the package contents, then the legal waters become much murkier.