On Sage 50/Instant, when in the Net box you could click on a little calculator button (or press F4) to bring up a calculator that allowed you to quickly make changes to an invoice total without moving away from the program. The calculated total would then get entered into the Net box. Very handy for when an invoice had to be broken up over multiple nominal codes.
Also add a Copy Line above option that Sage also had (by pressing F6). In journals, it pretty poor you have to use the Windows Ctrl+C and Ctrl+V to cut and paste text.