I have replied to the ticket but I also wanted to add what I discovered here. From my testing it looks like the shipping is failing. When I check on the confirm page I can see that the set session value code is triggered on any form post. What I think is happening is that when the order is submitted and the confirm page posts back to itself the form value is not present so the session variable is getting overwritten with a blank value. Further down in the code on this page the shipping quote server behavior is being triggered and failing because there is no value selected.
To update this you need to edit the set session value server behavior and set the trigger to the posted form element with the lightning bolt icon. This should get it working.