Add 15% tax on top of your purchase price. You don't have any way to avoid it unless it is a private seller and the seller agrees to claim a low price on shipping document.
If you ship by Air, the chance for being taxed is much less than when you ship by Ground. However, Most of the US online stores ship by UPS Canada Standard Ground, which is the cheapest with tracking number, so look at the UPS brokerage fee chart at the link below.