If I ship just a handgun lower assembly to Talon Grips, do I have to declare the package contains a firearm and also does it have to be sent via overnight mail? Thanks.

The handgun lower is the part with the serial, so in the eyes of the law, it is a handgun. So, yes. Follow all procedure and protocol as if you were shipping a full handgun.

And do NOT use USPS for it.  Non-licensees may not legally use USPS to mail what they consider handguns.  The USPS regulations define what is a handgun in a rather odd fashion and uses a test of concealability.  If you can conceal the firearm on your person, you must be an FFL to ship it using their service.  As a non-licensee, you will need to use either UPS or FedEx, both of which require you to use Next Day service.

Got it. Thanks for all the quick responses. I will send via UPS overnight and advise there is a firearm in the package.