When you added the service type did you also add in a session variable to hold this value? If not you should add in a set session variable server behavior on your confirm page and set the value of that session variable to the posted value from your service list.
Once you have done that you should also add in a new hidden form element to the form and give it the same name as the select list on the checkout page. You will then set the value of this hidden form element to the session variable you just created.
I think this would fix the problem because it seems that you are referencing the posted value of the service directly in the shipping server behavior. The problem is that when you confirm the order the values from the form on this confirm page are posted and the service is missing, adding in the hidden form element and setting it's value to the value of the session variable should ensure that the value is posted to the page again.