I know the law well on long guns and it does not apply in the case of a private sale other than the shipper must be notified they are shipping a long gun. State laws may require additional paperwork.
OK.
(a) It shall be unlawful—
(3) for any person, other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector to transport into or receive in the State where he resides (or if the person is a corporation or other business entity, the State where it maintains a place of business) any firearm purchased or otherwise obtained by such person outside that State, except that this paragraph (A) shall not preclude any person who lawfully acquires a firearm by bequest or intestate succession in a State other than his State of residence from transporting the firearm into or receiving it in that State, if it is lawful for such person to purchase or possess such firearm in that State, (B) shall not apply to the transportation or receipt of a firearm obtained in conformity with subsection (b)(3) of this section, and (C) shall not apply to the transportation of any firearm acquired in any State prior to the effective date of this chapter;
"Subsection (b)(3)" allows an exemption to this prohibition, which is the purchase of a long gun, made in person, from a licensee (FFL).
I was an FFL/SOT for more than a decade. I do not enjoy telling you, "you are wrong," but you are.