HTS heading 9803

Substantial containers and holders, if products of the United States (including shooks and staves of United States production when returned as boxes or barrels containing merchandise), or if of foreign production and previously imported and duty (if any) thereon paid, or if of a class specified by the Secretary of the Treasury as instruments of international traffic, repair components for containers of foreign production which are instruments of international traffic, and accessories and equipment for such containers, whether the accessories and equipment are imported with a container to be reexported separately or with another container, or imported separately to be reexported with a container — 1 code under this heading, with the duty on each.

Source: Harmonized Tariff Schedule of the United States, U.S. International Trade Commission (public domain). Classification and the duties that finally apply to a shipment are a licensed customs broker's call.