To assume Canadian buyers only look on .ca and US buyers on .com is foolish. You should allow Canadian buyers to give you bigger dollars and pay for more expensive "free shipping". You'll make more money that way.
There's no rule against saying "no tax", as long as the item is less than $800, there'll be no tax entering the USA.
My cousin has an online business selling mostly into the USA. His web site has "no state tax" in big font.