First, let me say thanks for releasing this plugin. The timing couldn't have been better, I was just about to start building this exact functionality for a site I'm working on.
One issue that should be fixed right away: It's setup to use secure2.convio.net, but some accounts, mine included, use secure3.convio.net. According to their documentation some accounts use a custom domain. Can you please add another field to the API setup screen with the server URL.