You are posting it up to your default calendar only:
postUri = new Uri("http://www.google.com/calendar" + "/feeds/default/private/full");
You need to set the correct Uri for posting it to another calendar.
Not sure, but I think it should be something like:
postUri = new Uri("http://www.google.com/calendar" + "/feeds/MYNEWCLAENDAR/private/full");