Is the value of the item (before shipping) more than $20?
If you want no surprises, look for US sellers who use the Global Shipping Program. This will compel you to pay the applicable duty and tax, as well as a hefty customs brokerage fee to Pitney Bowes who run the program. There may also be a short delay in delivery, because your seller sends the item to PB who then send it on to you.
But no surprises, eh?
Or you can check with Canada Border Services Agency about the duty that is charged on this kind of item. If it is made in the USA (or Mexico orChile) there will actually be no duty.
There will be PST/GST/HST depending on your province.
If the seller uses USPS/Canada Post for shipping there will also be a $10 service fee.
If the seller uses a courier like UPS the fee will start around $25.
So start by asking your seller how he plans to ship. Try for the USPS/Canada Post method which is usually the cheapest for you.