The Customs Duty Rate is a percentage. This percentage is determined by the total purchased value of the article(s) paid at a foreign country and not basd on factors such as quality, size, or weight. The Harmonized Tariff System (HTS) provides duty rates for virtually every existing item. CBP uses the Harmonized Tariff Schedule of the United States Annotated (HTSUS), which is a reference manual that the provides the applicable tariff rates and statistical categories for all merchandise imported into the U.S.
Duty-Free Shop articles sold in a Customs duty-free shop are free only for the country in which that shop is located. Therefore, if your acquired articles exceed your personal exemption/allowance, the articles you purchased in Customs duty-free shop, whether in the United States or abroad, will be subject to Customs duty upon entering your destination country. Articles purchased in a American Customs duty-free shop are also subject to U.S. Customs duty if you bring them into the United States. For example, if you buy alcoholic beverages in a Customs duty-free shop in New York before entering Canada and then bring thme back into the United States, they will be subject to Customs duty and Internal Revenue Service tax (IRT).