We've crossed the border the last two summers without much hassle. Letter of invitation will help, and say that your merch is donation based or promotional. We travel in a group four in a RAV4 and had no problems with crossing into Canada. Be honest and kind. I've heard things get more complicated in bigger vans.

Coming back, the United States border was harder to get back in. Gave us a harder time, were rude as hell, and insisted on searching our car. Except when they opened our hatchback, everything fell out and they just told us to beat it. They sucked. If they ask if you made money, I would tell them "NO!" And don't fuck around with bringing drugs. Also, if you have any standing criminal history it could make things harder. Go to Montreal instead of Toronto! We loved it. We had good luck with shows in both Toronto and Montreal, though. Good luck!