No requirement of any State or territory of the United States, or any subdivision thereof, or the District of Columbia, with respect to bonding of packers or prompt payment by packers for livestock purchases may be enforced upon any packer operating in compliance with the bonding provisions under section 204 of this title, and prompt payment provisions of section 228b of this title, respectively: Provided, That this section shall not preclude a State from enforcing a requirement, with respect to payment for livestock purchased by a packer at a stockyard subject to this chapter, which is not in conflict with this chapter or regulations thereunder: Provided further, That this section shall not preclude a State from enforcing State law or regulations with respect to any packer not subject to this chapter or section 204 of this title.