Ok, there are different forms if the item goes beyond $200 (and another one if beyond $2000). A lot of stuff is still duty free under $2000 - but the major deciding factor is country of origin ... free trade. You'll have look it up on their website.
It should be noted, for now, it is the person who is recieving the goods responsibility to pay the state tax on the goods (if it is over their personal excemption). This may change in the future.
Todd, I have the invoices - like I say they rarely want them ... just the customs sheet. Works out good, you need the invoices for your records anyway. Weights are not needed, just the price. You should get a decal ... it's $100US and good for the rest of the calander year - then you don't have to pay the $5 each time.
On the Global Priority - I do fill in an amount. Canada is just like any other country I ship to ... it is up to the PO to collect taxes.
If I am shipping over $200 US to a person not in the US I just use CP - the customs guys don't care if you are shipping to another country (only that you are bringing it into the US) ... you have to fill out the other forms, check excemptions, etc.
Todd, the eBay invoice is fine (If it has the name, address, amount and description). I use my own invoices (for the most part automatically created) for many reasons. Most of the reasons are because I need to track a transaction not just an eBay auction.
