I have asked around and it seems that this info is not available in any guides. I can give you some pointers though.
The idea is that the user will select the service type for the provider you are using by making their selection from a list on the checkout page. This posted value from this list should be saved in a session variable. You will then reference that session variable in the shipping quote server behavior.
How you would start this is by opening up your checkout page and adding in the service select list. The code for the select list can be found in the snippets tab of the file panel. In here select WA eCart then select shipping, from this list choose the service types for you shipping provider. This is the code for the select list, you can just copy and paste it into your checkout form.
Next, on the confirm page you will add a set session value server behavior. Set the trigger to the posted service select list, then for the value select the service select list.
The final part is to edit the shipping quote server behavior and use the lightning bolt icon to select the session variable you just created for the service selection.
This should allow you to make use of the service selection that the user made in your shipping server behavior.
