I want to give Free Shipping only to Prepaid orders.
For all COD orders, I want to charge a shipping fee.Please tell me how we can do this on the backend? Is there an app that I can use for this?
You can set this under the "Shipping & Delivery" settings. For the zones you have setup, you can add flat rate as shown in the below example.
Once you do this, something like this will be displayed at checkout.
I hope this helps!