Brian I also live in Canada and have been hiking the JMT and Sierra for the last six years. I take all my own food, and have found the best way to get resupply boxes across the Canadian/USA border is to drive across by car with the boxes unsealed. Then I take them to an American Post Office and mail them to myself. I have had no problems driving
across the border with the boxes. I have flown to the USA with resupplies, again no problem, just expect more questioning by the USA border people at the airport.
I have had luck sending resupply packages via UPS, but this is expensive, and can take
longer than you would expect.
The culture of backpacking in the USA is much stronger than in Canada. You will find it
much easier to purchase resupplies along the PCT, than you would expect in Canada. Getting fuel is easy, check Yogi's PCT Handbook for details on where to find fuel and other supplies. Another source of resupply is to order food from companies in the USA
(REI, Just Tomatoes) and have them ship directly to a resupply address ie: post office or motel.
You are welcome to send me a private email on any details.