If it bothers you to do it in person or you feel like they might convince you otherwise despite your intentions, just send an email to all parties involved. As far as contracting goes, I don't think anyone has any grounds to get mad (though they might). It is the nature of contracting. That is why you are contracting and not working full time (although really, in the US, it is the nature of pretty much all work--at will employment). Be polite.

If it helps, remember this: they would absolutely not hesitate to let you go with less notice than you plan to give them, oftentimes not for performance or anything negative but simply because the company is cutting costs. This is your version of "cutting costs."