Actually, I'm a bit of an expert on shipping from the US to Canada having dealt with hundreds of shipments over the years. Some of the posts in this link are accurate, others are patent nonsense.
The short version:
Post Office - slow, hard on packages, poor tracking, cheapest brokerage fees.
FEDEX Express- the best. Fast, reliable, great tracking, no brokerage fees but VERY expensive.
FEDEX Ground - next best. As above but cheaper and slower. Minimal brokerage fees.
UPS- cheap up front. Expensive at the door. Reliable. Good tracking. Ridiculous, bordering on usurious brokerage fees. Class action lawsuit, anyone? Definitely your least cost effective choice.
All carriers charge Federal and Provincial taxes or duties as required. The Post Office is the least efficient tax collector. I have had packages worth $500 US arrive at the door - no charge, due, I suppose, to inefficient data control.
AS for our US friend's assertion that the shipper MUST warn buyers of duties and taxes - this is absurd! It is incumbent on the buyer to know the laws in his home country. A shipper can't be expected to know the regulations in Zimbabwe or Sri Lanka!