Shipping Cost Per Product


I can add additional shipping cost per product on Shipping Properties of the product (minus or plus).

But when I add product with shipping surcharge/discount and other product with ordinary shipping price the system still show the first shipping price in the cart. Is it the way it should work?

For example:

All products have 1 shipping method (Standard 10 USD)

One particular item has shipping discount 4 USD. (Shipping freight: -4 )

So when I add this item to the cart the shipping cost shows 6 USD. (that is OK).

But when I add another item with standard shipping methods the system still shows the 6 USD cost instead of 10 USD

It "should" be summing the surcharges of the products in the cart and adding that to the calculated shipping price of all products related to that shipping method.