Under the CSA, a nonnarcotic controlled substance cannot be imported unless it “is imported for medical, scientific, or other legitimate uses, and is imported pursuant to such notification, or declaration, or . . . as the Attorney General may by regulation prescribe.” 21 U.S.C. § 952(b). The Drug Enforcement Administration (“DEA”), as delegated by the Attorney General, has promulgated regulations requiring importers of such substances to be registered and file an import declaration.