In the US, USPS (ie, the post office) is generally the best best for this type of thing. Fedex/UPS/etc will all do it as well, but in my experience, USPS is cheaper. You can do all of the paperwork via their website.

I'm guessing you're shipping from Canada, so I'd start with CanadaPost as I'd guess they have similar functionality.

For a (relatively) low value, used item there should be no duty payable.