re: Would you sue over $400 or write it off?

It's not about the money anymore. I would phone his office incessantly, email him invoices every day, and if that fails I would take out an ad in the Picayune displaying your invoice, number of times contacted and ignored, and a note at the bottom explaining you are a young businessman and simply want what he agreed to( I would provide copies of his email agreements as well). I wouldn't care if this cost me well over a thousand. Do Unto Others.... but that's just me.