Ad valorem tariff.

A duty stated as a percentage of the customs value, so it moves with what the goods are worth. The alternative is a specific duty, a fixed amount per unit of quantity (per kilogram, per dozen, per litre), which ignores price entirely, and some tariff lines carry one of each stacked together. Apparel is mostly ad valorem: the HTSUS general rate on knitted cotton T-shirts, subheading 6109.10.00, is 16.5% (USITC Harmonized Tariff Schedule). Because the base is a declared value, the figure a shop writes on the commercial invoice is a duty decision rather than paperwork, and customs reads it that way. On an inbound pallet of blanks the value is what the shop paid the supplier, not what the shirts will sell for after printing, and getting that wrong scales straight into the duty line because the rate is a percentage of it.