Indeed, the customs forms were a pain. There was no way to track or insure the package I sent...
The one time I did ship to Canada took two separate trips to the post office and a lot of line standing with DD4 in tow. (Not so much fun!)
At the time, I was happy to ship the item to Canada, not having done shipping there before this.
Having done it.. I don't care to do it again.
Shipping only to US addresses allows me to pay and print my postage from my computer and toss the package in the mailbox all from home.
I lived in Toronto, ON for just over a year (1997-1998) and can sympathize with those that live there (Canada) regarding the availability of some things.