Hi and welcome to our site!Customs Duty is a tariff or tax imposed on goods when transported across international borders.
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 based 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. The Harmonized Tariff Schedule may be found here - http://www.usitc.gov/publications/docs/tata/hts/bychapter/1301htsa.pdf
As you see - Customs Duty is based on classification. Based on your explanation - Electronics Components is duty free,however - there will be co-called Merchandise Processing Fee ~$35 when custom declaration is filed.Because the Harmonized Tariff Schedule is a very large publication - you might want to use this online version - http://www.dutycalculator.com/new-import-duty-and-tax-calculation/