My understanding is that it depends on the country of manufacture.
If it was made in the US, Canada or Mexico then there is no duty (NAFTA).
There are other countries that we have free-trade agreements with but
you have to know which items imported from those countries are eligible
for reduced-duty or duty-free. You should be able to get more info here:
http://www.cbp.gov/xp/cgov/import/duty_rates/
Duty IS based on the value of the item and an invoice should
accompany the shipment.