In practice
Four of those in more detail
How to find your HTS code
Start at the Harmonized Tariff Schedule published by the US International Trade Commission, which is the actual schedule rather than a copy. Work down: chapter, then heading, then subheading, reading the section and chapter notes as you go because they exclude things that would otherwise seem to fit. Then search CROSS, CBP’s ruling database, for your product type and read how CBP reasoned about similar articles. Two products that look identical on a shelf can sit in different chapters because of what they are made of or what they principally do.
What you should not do is take a code from your supplier's invoice and file it. The supplier classified it for export from their country, under their own schedule, with no exposure to whether it is right for yours. If you will import the product repeatedly, convert the question into a settled fact with a binding ruling request.
Tariff engineering, and where the line is
Designing a product so that it lawfully falls into a different classification is long-established and legitimate. The classic examples are real: a garment's fibre blend crossing a threshold, a footwear upper's material proportion, whether an article is imported finished or requires further processing. Companies have done this openly for a century and the courts have repeatedly upheld it.
The condition is that the article as actually imported has to genuinely be what the lower code describes. Engineering the product is lawful. Describing it as something it is not, or making a cosmetic change purely to mislead while the commercial article is unchanged, is misclassification. The distinction is whether a knowledgeable person examining the goods at the port would agree with your description.
The first sale rule
Known in the trade as the first sale rule. Where goods pass through a middleman before reaching you, the customs value is normally the price you paid. In a qualifying multi-tier transaction it can instead be the price in the earlier sale, from the factory to the middleman, provided that sale was genuinely for export to the United States, was an arm’s-length transaction, and you can document it.
The saving is on the value everything else is calculated from, so it reduces the duty, and it compounds across every trade remedy stacked on top. The requirements are strict and documentary, and the documentation has to exist at the time rather than be assembled afterwards. This is worth a conversation with a customs professional if you buy through a trading company at any volume.
Assists: the value you supply that becomes dutiable
If you provide the factory with something free or below cost that is used to produce your goods, its value is generally added to the customs value. The common ones are tooling and molds you paid for separately, materials or components you supplied, and design, engineering or artwork developed outside the United States.
This catches people because none of it feels like part of the purchase price, and a mold invoiced as its own line looks like a capital item rather than an addition to the value of every unit it produces. The apportionment method is something you choose and document. Getting it wrong understates the value on every entry, which is the kind of error that compounds quietly until an audit finds all of it at once.