The courier companies do their own customs clearance, and as a result, have to charge duties, GST and PST, and then charge for the honor of doing so. Often these charges are substantially higher than the actual shipping charges.
In addition, all the couriers use a zone system, so if you ship from the US West Coast to PEI, or NS, look out. Post charges are not zoned, all of Canada is one price, no matter how remote.
The reality is Postal shipments can be charged, but rarely are, and the fees charged by customs are a fraction of what Fedex, DHL or UPS charge for the honor. Canada Customs just cannot be bothered most of the time.
When I ship to Canada, I usually point out that even Air Parcel Post to Canada tends to be less expensive the UPS ground. The shipping isn't what kills you, it is the destination charges that get tacked on!!