Assign free shipping for the whole cart

I have an online store selling multiple items, some of which are big and the shipping fee is unknown, and need on-delivery-shipping-fee-payment; as a result the customer should not pay any shipping fee prior to shipping their package.
So I need the plugin to consider this matter when such items are included in the cart among other items, no shipping fee should be added to the total, since they are all sent as a whole package.
I have tested the plugin and as long as there is one item , there is no shipping fee inclided. but the moment I add some other items, it adds the shipping fee!

 

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