A simple option is to use a pop up menu in place of a drop down list. (And you don't need a script and trigger to copy over the description and percent values over. You can set up looked up value auto enter field options to enable this to happen without a script being used.
The pop up menu's "beveled block" look can be hidden if you don't like it's looks.
The following demo file demonstrates several ehanced value selection methods. The drop down in the line items portal row will do what you describe. The item number fields are included in the layout so that you can watch things work, they can be removed from the portals without affecting the function of any of these methods.