invoice modification

We do a lot of consulting. Some of our billings are for fees, not products. I looking at the invoice functions, it does not appear that one can modify the invoice structure to work with fees, as the 'product details' field cannot be removed or modify. Thus, the invoice shows up with Product Name, Quantity, Unit Price, List Price etc., which are not appropriate for consulting fee billings. Did I miss something or is this set up for product-based invoicing only?

Thanks