I sent my transcripts and proof of citizenship in a couple weeks ago, and when I called to ask about the options, they specifically told me to snail mail it. That being said, last year when I applied, they told me I could fax the stuff in. I did that, but due to something being set wrong on my end, the fax ended up being essentially too low quality to use. If you aren't sure, just give them a call and ask. They'll give you a definitive answer.
One lesson I learned the hard way: don't bother overnighting your stuff. I was so eager to get my stuff in that I paid $26 bucks to send it overnight via Canada Post from Mississauga, Ontario to North Bay. It made no difference, and in fact I was told that when things are sent to them by courier, they have to be picked at the post office, which only gets done once or twice a week anyways. So if you're going the mail route, save your money and just go with a good ol' stamp. In the end it might actually be faster!
