Order Entry Form

I recommend to check invoice template for this purpose:

http://xlinesoft.com/invoice

Thanks I did and I see something somehow similar to what I need. I wish I could be able to do it out of the box.
I have a couple of things that are holding me back, because I don't know if the template can allow the bellow modifications.

1. My tax is linear (per item), not calculated at the end
   
2. Instead of searching items in each line of the invoice, I would probably do the item selection as part of the header, then once the item is selected, the quantity and price entered, I would press the add button so it will be added to the list of items, This way on a large table with many items, I can filter them by group, then by item (as chained drop downs).
   
3. Can I make these changes in the template?
   
4. Live demo 3: Using dropdown box ordered by priority; is not working properly.

Invoice template is just a template meaning you can create a project using this template and make any changes later on. You can add new tables, fields, add new functionality or modify any existing one.