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

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,
Spencer

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