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,

toolbox149
As far as I know, there are no duty or custom fees on product shipped from Canada > USA, only duty fees on product shipping from USA >Canada.

The seller will have to fill out custom forms to ship, but there should not be any extra fees on your end. 
have bought several things on Canuck Audio Mart well over $1000.00us and never had any issue with any applied duties or taxes.
Thank for the help.  
Is there a form that needs to be filled out to get a parcel through customs, and if so, can I get a copy from somewhere so I can send it to a seller prior to the seller getting to the post office?