Re: film processing mailers
I would think that any of the U.S. labs that do prepaid mailers (such as A&I - the ones sold by B&H) 'should' be able to send your prints or slides to Canada without a problem - albeit for an additional postage charge, of course. You would need to check with the company for their policy, of course.
And you would also have to pay the outbound postage on the mailer itself, as I'm pretty sure Canada Post doesn't honor the U.S. Prepaid postage on the mailer envelopes.
A better solution would be to find a supplier within Canada, which may take a little digging, just as it often does here. Remember, GOOGLE is your friend!
Re: film processing mailers
Looks like Scala has a special mailer. See it here. Not many places in North America process it so it's probably the best bet. Another option is the DR5 process. Not having done it myself I can't really recommend it, but looks interesting. Here's their website.
I've used the Kodak mailers before, and it took 2-3 weeks total. That's been a few years, but the amount of time it takes probably hasn't changed. Not sure either about shipping to Canada though.