How can i auto populate Informational cost on Buy shipment using rate record

We are just using Buys shipments for one of our client. But now, they have a requirement where they want shipment cost to represent what they actually they need to pay to carrier (i.e. cost on which our line tolerance rules would get applied) and want to have one other cost which they want to show to customer.

I understand this is something which can be achieved if we use sell shipments for same order, but i was thinking of avoiding sell shipments (because we already have load in our system due to high volume of orders everyday and we would need additional maintenance to maintain all master records like itinerary, rates etc related to sell shipments).

So, i was thinking of additional that information as "informational cost(display)" in the buy shipment financials tab. But i am not able to find a way to populate that cost automatically using the rate record.

Wondering if someone can suggest me a way to achieve this?

Thanks for your kind attention !!