Right, I had forgotten about that completely - that you can subtotal at the end of the invoice and get a tally of everything on the invoice. My only issue with that is that when you view or print that invoice, that subtotal line is mushed in right below all the other lines and it can be difficult to work out that it's a subtotal and not another line in the invoice. I suppose you could space down a few lines and do it lower on the invoice, which I think would be the only workaround at this point.
Thanks for taking my query to the development team . . . it just seems really odd to me that the checkbox for the 'screen' Subtotal is already there, but is unusable. I'm not sure what the reasoning was behind that move, but I hope that they enable the 'screen' checkbox because life is just better when it's visible in your working screen. :)