UPS & al rip off Canadians for sure - US Postal Service/Canada Post works great - IF and only IF your vendor agrees to use them. UPS has incredibly good rates for the US only... they must have a gold plated building in Fort Erie, ON, with all the brokerage fees they've charged.
Other wise, I plan my purchases around my US camping trips and pick it up over there.
Finally, if you have a friend in the US, have it shipped there and then have that person send it to you by USPS, if that is possible. Not too often though, as you may not have that friend for long