My only experience is with the USPS, with shipments from about half-a-dozen of different U.S. suppliers. The experience is generally about the same, extra charges at the Canadian Post Office for GST, PST and a small handling fee. Two shipments did not have any extra fees, with one of those shipments marked as "gift". Most suppliers will not mark as "gift", so it was much appreciated when I discovered it. So far, it takes about 1.5 weeks to arrive at my doorstep after the order has been sent.
It normally isn't too much of a problem, except when I don't plan my jobs very well and I have to order more parts,

then the shipping starts to add up.
I know people who have had shipments sent by the "couriers" like UPS, Fedex, etc. and it always seems to be the same crap shoot when it comes to trying to guess what the fee will be. I would think that it would be more predictable and up-front, seeing it is the business they are in, but instead they choose the extortion method when they show up on your doorstep.
I don't have access to the border.