forms for bringing parts across the border

Does anyone know the correct form to fill when bringing parts into the U.S from Canada? Im talking about the one they would nail you for whatever fee at the border when bringing it across yourself to ship to someone you sold it to.

Where can I download it?
You don't need any forms. If the company sends it UPS then UPS makes you pay all the government fees. If the parts come from a private person, the customs will look it over and send you a "please pay now" in the mail. If it comes by mail from a company, then the post office collects the government fees when you go to pick it up. I have found no legal way to keep the government from robbing you when importing parts.