Here's a down load link to a demo file that uses 4 different enhanced value selection techniques to add line item records to a portal on an invoice. Any one of them can do what you want, but the drop down located in the description field of the portal row is probably the best match for what you describe here.
Wow. How depressing. They should call this product "ScriptMaker" instead. I feel like all I am doing is scripting anymore.
One of the most useful layout tools in my opinion is the drop down menu. I can't believe it is not more straight forward.
Anyway, thank you for your very informative response. I just wish there was an easier way.