You never tell the postal clerk what is iin the package, it's not required to. You insure it and then the pick up person has to sign for the package verifying it was delivered.
You do not write on the box it contains a firearm either, if you do you're asking for it to be stolen.
I shipped hundreds of firearms this way.
My transfer fee is $30.00 which includes a trigger local and sales tax.
If you purchase from me direct, their is no additional NICS fee.