I modified the starter Invoice Solution to do what you were asking. You can download the sample file from the following address:
You select the Category at the top of the Invoice then when you select the Item from line item it shows only the items from that category. You have to change the category on each line item if it is in a different category.
I had to make update. The category had to be added to the portal to work with every line item.
Thanks S Chamblee!
That was perfect and easy to undertand. Very much appreciated. So many more problems will solve themselves by seeing your example.