you can either load the last letter as a separate field in the script (>> RIGHT([Freight Code], 1 <<)
or you can create a new listbox_object on the GUI using this same formula.
Voila, you can select > S < in this listbox and all the Freight_codes with an S at the end will be selected for you.
P.S.: Of course, you can also create a flag_field that will show 1 or 0, either in the script or on the GUI.