As spijker already suggested, my plan to ship the film by Fedex isn't going to be as simple as I thought. Crossing international boundaries introduces a whole new set of issues (and costs).

So, I have the option of bringing the film with me and then taking it back with me, or buying the film in Toronto and carrying it back to the U.S. when I leave. Which is easier? I'm expecting to shoot about 20 rolls or so of 35mm. I roll my own, so it would be much cheaper to bring it with me from home. Can I declare it on the Certificate of Registration For Personal Effects Taken Abroad (suggested by Marc B) the same as camera equipment?

If I buy film in Toronto, do I have to pay a duty on it when I come back to the U.S.?

I have heard horror stories about transporting film in checked baggage, so I assume I will need to take in a carry-on bag of some kind and send it through the x-ray. I'm not thrilled at the prospect of carrying a large-ish bag of film along with all my camera equipment around with me and I don't trust their claims about x-rays not hurting film, but it sounds like that's going to have to be the plan.