It probably is easier to do it as a surcharge.
Just update all of your rules to not have a condition based on the "ShipFlat" column.
Then add a rule based on a multiple of the ShipFlat column to add the surcharge.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.