What do I need for customs if purchased in Canada & ship to US?

If I buy a component from someone in Canada and want them to ship it to me in the US, what should I have the seller do to get it through customs?  I'll probably tell them to use the Canadian Mail to USPS.  
Are there forms to be filled out & maybe attached to the outside of the box?  A copy of the receipt attached to the box?   Something else.  My total purchase amount would probably be $1,000 or less.
How expensive are the US customs rates?
Thanks,