Besides the invoice itself some other questions need to be answered.
-Are you using any kind of item database or inventory software which the invoice is systematically pulling data from?
-Are you planning to fill out the invoice electronically using a keyboard or by pen/pencil?
The answers really depends on the level you're looking to get into and can range from something as simple like a receipt you'd receive at a flea market to an invoice on a cutting edge e-commerce site.
I'm assuming you're looking for some kind of free-form template which will automatically update the total (and maybe tax too) as you add items to it?
EDIT: Found these in a quick google for 'invoice templates': http://office.microsoft.com/en-us/t...0117255.aspx