I'm Canadian, so I'll just jump in to confirm what others are saying.
UPS sucks, avoid if at all possible. They can hit you with ridiculous charges as others have said. And I've had problems when I wasn't home to receive the package. Like the last time (and it will be the last time) I had stuff shipped by UPS, they actually gave it to someone else who lives in my building. I don't think they're even supposed to do that unless you tell them to. Oh yeah, and they just left the slip on the wall in the front entrance, so anybody could have taken it, said they were me, and picked up the package from my neighbor. I was pissed because there are a few sketchy characters in my building and I was afraid of who I might have to talk to to get my package and that I may not even get it all. I got lucky and it turned out the person who had it was a nice older lady, so all's well that ends well I guess, but I will never deal with UPS again.
DHL is ok, never had any problems with them.
USPS handing off to Canada Post is my preferred method for ordering from the States. Price should be reasonable and should be hassle free.
I've ordered from divemaster, GGG, and Ultimarc and was pleased with the service from all of them.
I ordered from Playdium and while their prices seemed ok to me and their service was great, I thought the shipping was kinda high and they shipped UPS, so I won't order from them again.
To sum up, probably can't go wrong with any of the well known vendors around here, just make sure they're not shipping UPS!