For configuring the COD - Extra Charges Extension, admin can go to Stores > Configuration > Sales > Payment Methods > Cash On Delivery Payment.
For this extension to work, an admin needs to enable the Cash on Delivery Payment. Admin can change the title and instructions of the message which will be displayed on the front end. Along with this, admin can choose the selected countries for which the charges are applicable.
This extension enables the admin to charge on COD either on the order items or order subtotal.
If you choose the option ‘Order Items’, the charges will be applicable on bases of various parameters like price, SKU, category or custom filters.