Would it be possible for you to show some example invoice (maybe an edited SubTotal invoice) that shows where and how the total should be displayed in your opinion?
The main problem I see with implementing this feature is that SubTotal doesn't "know" what is being invoiced: services (e.g. 20 hours of work) or goods (e.g. 5 liters of paint color). And you can mix these in one invoice (20 hours of work + 5 liters paint color). Those can't just be added up to "25 hours"