first, you would need to add the shipping server behavior to the confirm page. Open the confirm page, go to the server behavior panel, click the plus button and select eCart -> Shipping to get the list of service providers you can add.
then you need to add the shipping rule to the eCart object shipping panel.
Set the trigger to:
Based on [Selected Shipping Provider name] Quote value > a value of 0
set the calculation to :
Based on [Selected Shipping Provider name] Quote value