This doesn't add anything but I once mailed a firearm via USPS. I had a neighbor that was retired from the PO and he told me exactly what to do and ask for. I don't remember the details now, but he advised me to send it a certain way so that it is stored in a safe whenever it is not in transit. At each point in the journey it is removed from the truck and signed for and secured in a safe.
The gun was fully disassembled. In fact, I didn't even ship most of the parts. No inspection was made. And they didn't bat an eye when I insured it for $8000.
It was a total non-event. They acted like this is done all the time and showed no reaction at any point. I should mention that this was in Nevada.
You know the rest. In the books you have read
How the British Regulars fired and fled,
How the farmers gave them ball for ball,
From behind each fence and farmyard wall,
Chasing the redcoats down the lane,
Then crossing the fields to emerge again
Under the trees at the turn of the road,
And only pausing to fire and load.