No script needed.

"I'm hoping some smart guys knows how to do this."  - He nailed that guess. 

Can I accumulate into a standard Sage Field (FreightAmt) or does it have to be a UDF?

Thanks.

That feature is for a header UDF, but you can add a script to copy that value from the UDF into FreightAmt. 

Note: depending on module options, Sage may overwrite FreightAmt values with a calculation based on shipping rate maintenance settings for the ShipVia code.