In that case it is all passed in those hidden form elements. They may already be there... look for x_shipping (or something similar I don't remember exactly and don't have the Authnet documentation in front of me)... All you should have to do is set the value for that hidden field... if it doesn't exist, you can look up the correct name in the authorize.net documentation and just add it. Use the bindings panel to set the value from the cart shipping total.
Did this help? Tips are appreciated...
