Any misbranded hazardous substance or banned hazardous substance when introduced into or while in interstate commerce or while held for sale (whether or not the first sale) after shipment in interstate commerce, or which may not, under the provisions of section 1263(f) of this title, be introduced into interstate commerce, or which has been manufactured in violation of section 1263(g) of this title, shall be liable to be proceeded against while in interstate commerce or at any time thereafter, on libel of information and condemned in any district court in the United States within the jurisdiction of which the hazardous substance is found: Provided, That this section shall not apply to a hazardous substance intended for export to any foreign country if it (1) is in a package branded in accordance with the specifications of the foreign purchaser, (2) is labeled in accordance with the laws of the foreign country, and (3) is labeled on the outside of the shipping package to show that it is intended for export, and (4) is so exported.