Shipping cost to vendors buy not to cutomers

We use WCFM Marketplace for out store which has around 20 vendors currently. We do shipping ourselves for all the orders, means we pick up from vendor and deliver to the customer. The price which we display to customer includes shipping price aswell as we promote it as free shipping,

My question is that I want to deduct shipping charges from vendors earnings but on cart to customers it should show free shipping. How can that be done?

Our shipping cost is weight based, so we cant use fixed fee field. So wanted to ask how can we do it so that it remains free shipping to customers and weight based deduction to sellers