If you're selling it thru the FFL, they will require a copy of your driver's license to be included.
You can call the FFL to be sure what they require.
Contact the FFL directly, period.
If mailing USPS, use Priority Mail (not Express), insure for over the cost, signature required.
Tracking number to the FFL (if they request) and the buyer.
You do not have to declare it as a firearm.