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

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.  

This is why i buy in USA only.Good luck.