You'll probably enjoy this link:
I also live in Canada and have performed numerous trades via the mail. My best advice is to go Canada Post, its still not legal but I've used other shipping companies and they've either broken bottles and not delivered or, in the case of UPS, twice had the package's contents stolen though the box arrived.
Shipping across the border is a different issue altogether. I've found that American customs really don't care about checking the content provided there is something stating what it is. Canada Customs opens 50% of the inbound boxes from the USA because they like to perform paperwork and tax every items worth.
If I'm shipping to the USA, I like to combine my shipment with a trip across the border... after all, its nice to take the opportunity to check-out what's new in the bordering state, pick-up a couple of bottles of Full Sail, Stone or whatever... send off the package to a friend via US Postal Service and have avoided the Border Police in nabbing my package.
I usually put the beers, a bottle-sized box (with shipping tape, a window scraper, gloves, scarf, etc) and the newspaper in the trunk of the car that I claim are for personal consumption or "I forgot about them being in the trunk". Then when you get into the USA stop at a nearby post office and package it up in the backseat of your car by rolling the bottles in the newspaper, taping them then putting them in the box. Simple.
Have fun... beermailing if FUN!