I agree with everyone that UPS and Fed Ex suck ass.
All good advice here except:
Basically, most items coming from u.s. are duty free. So, i don't think you have to pay any duties. If the item was shipped thru fedex or ups, you will have to pay a 30-40 dollar brokage fee. If the item is shipped thru usps, you may or may not have to pay a 5 dollar fee to custom (it depends on whether custom inspect the item).
whether you have to pay taxes depends on whether the seller is registered to collect taxes. some sellers will label item as a gift or understate the value so you do not have to pay taxes, but this is illegal. However, it happens on ebay.
This is pretty much all wrong (sorry).
You'll pay duty on certain things, covered here:
http://www.cbsa-asfc.gc.ca/general/publications/customs_tariff-e.html . Basically, anything manufactured outside the US and being shipped from the US will be charged a tariff, but there are exceptions. Tariffs vary (and are listed in the site above).
In addition, you'll pay GST and PST on almost all items, no matter if the seller is registered in their own state. You don't pay their state tax, you pay Canada's taxes. They don't usually charge on used goods, but technically then can. A $5 handling fee is tacked on in addition to the taxes and duty.
All this applies IF your package is stopped at the border. about 50% of my stuff goes through untouched, even big items. The best is if you can get the seller to mark the item as a gift. Then you don't have to worry about any of the above. The worse is when the seller overstates the value (instead of writing down the sale price, they write down the 'retail value'). Then you get dinged on the retail!
Also, if you do go through UPS or Fed EX, the brokerage fee is not a flat fee, it depends on weight of item.