Writeback of freight amount to GP

We are trying to write the freight amount back to GP 2013 using Shiprush SQL. The SQL script uses sample code with the following after decalrations and code to update tracking numner to comments which works fine

UPDATE SOP10100
SET FRTAMT = @shipcharge, ORFRTAMT = @shipcharge, DOCAMNT = DOCAMNT + @shipcharge
Where SOPNUMBE = @RecID