Besides that, I tend to just buy off tirerack and ship it to the border and pick it up. the other option is get it shipped from the US to your door and hope they dont use UPS. If they do, buy a bottle of KY jelly for when the UPS guy gets there and charges you the brokerage fees.
Last time I ordered from tirerack, they did all the customs paperwork up front, shipped UPS and all was well - no surprise bill from UPS.
As long as the retailer does the paperwork, you should be OK, but agreed that if they are too lazy and rely on UPS to broker it, definitely time to break out the anal lube.
Never tried tiretrends.ca, but I have done the tirerack thing two years ago when I got a set for my '04.
Could be they've changed their shipping protocol though, so don't quote me on that.
If memory serves, much like BlairTruck mentioned $60 per tire shipping from Discount Tire, tirerack ran about $50 per tire, and I still came out ahead.