Good intel.
B.S. My FFL does all my pistol transfers through USPS so I can save bucks.
Fact: If you want to ship something with intrinsic value (gold, guns, dope) ship it REGISTERED mail, not CERTIFIED.
REGISTERED MAIL is hand carried to it's destination, and when it is handed off, every person has to sign for it. It is the USPS premium service and it is really proud of it. The loss rate is minuscule. The postal inspectors jump on any loss of registered mail.
It is expensive, but something you should consider.
As to transfers, there are several categories of pistol transfers that can go USPS, it's right there in the Postal Mail Manual. Not a big secret,