I have a scenario like, a Customer will purchase a product with 25 Qty, but during the checkout time, the bill or cart subtotal is generated for 25 qty. Later while checkout, shipping charges(fedex services) will be added for the 25 Qty. But the thing is, the customer will select an option like, for now, say out of 25 qty (which is already billed), customer will opt for 15 qty to be shipped first. For this only 15 qty shipping charges should be applied, Not for the entire cart Qty.
I can not work out how to do this. Please share your valuable thoughts.