19 C.F.R. § 10.456

Accessories, spare parts or tools that form part of the good's standard accessories, spare parts or tools and are delivered with the good will be treated as a material used in the production of the good, if—

(a) The accessories, spare parts or tools are classified with and not invoiced separately from the good; and

(b) The quantities and value of the accessories, spare parts or tools are customary for the good.