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?

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?

When the seller buys the postage he completes the form. You shouldn't have to do a thing. The seller states what's in the box and what it's worth. Not a big deal. Cheers,
I had a bad experience using UPS to ship from Canada to the US. They billed me for customs, and imposed a brokerage service charge, for an item that should not have been subject to customs. It was for a Triplanar tonearm that I already owned. I sent it to the builder of my turntable so that he could drill the holes for mounting of the arm. UPS billed me for the return shipping of the tonearm, but not for the turntable. Weird. Since then, I insist on using Canada Post for items shipped from Canada. No problems with them.
Thank you to all!   This is a big help.   I intend on requesting that anything I buy be shipped Canadian Post or FedX.
Thanks again.

I bought a used B & W subwoofer from Canada....had it shipped via UPS to TX.  Unknown to both me and the seller, the import duties are assigned based on the country of manufacture.  Since the sub was in the original box and B & W products are made in China, I got hit with something like 5% tax...including some extra fees they helped themselves to it was about $150.  I wasn't that thrilled.  You can go to this website and fill in the parameters to see what kind of taxes you are looking at. https://www.dutycalculator.com
Any customs forms will be completed by the sender. If the item is light enough it should be sent through Canada Post. More than likely the item will enter USA with no problem. If the item is made in Canada there should be no duty, if not you COULD be accessed duty but probably not. Country of origin is filled in on the customs form.
I frequently buy and sell overseas and have paid duty and fees only once courtesy of UPS.
As mentioned in another post, it doesn’t matter where the item is shipped from, what’s important, in terms of customs duties, is where the item was made. If it was made anywhere other than in the countries covered by NAFTA, the North American Free Trade Agreement, customs duties may apply.

Having said that, the enforcement of those rules seems to be arbitrary.
Xpresspost USA and Expedited Parcel USA:

Name, address and phone number of sender
Name, address and phone number of recipient
Non-Delivery Instructions - (Return at Sender's Expense or Abandon)
Reason for Export (Gift, Sale,Document,Repair/Warranty,Commercial Sample, or Other)
Quantity,Detailed Description of Contents, Net weight, Declared Value in Canadian Dollars

I never bother with the net weight.  They do that anyway when I take it to the post office.  I also usually fill out the country of origin and HS Tariff No. from the Commercial Senders section to help avoid confusion.

I have no clue if any of my buyers have been dinged with duty, taxes, etc.  I just warn that it is a possibility.  I never send UPS or Fedex, as I am working under the assumption that they are doing the same sort of charges on CDA>US that they do on US>CDA

I recall when I bought a pair of Coincident speakers from Canada, UPS called and needed my SS#.  At first I thought it was a scam call, but I was told when anything over a certain value was shipped to the US the Patriot Act, or something like that, required the recipient's SS#.  Turned out it was true.  I don't know if it's still true.