Assign Sell Side Rate according to Buy Side Rate

Please, if anyone can write some lines on the following, it will be helpfull.

i think this might be a solved functionality but cannot find it.

SCENARIO:

As shipper, I create Buy side shipments and then want to charge my customers with freight amounts (let's say +10% of the freigh cost)

ISSUE:

We find that we can "build a sell side shipment from buy shipment", but the problem is that it goes and search for the better rate available (cheaper almot always) from the sell perspective.

We want the sell side shipment to automatically and only choose the very same buy rate ( that has to have included the additional amount for the sell-side perspective ).