It allows you to SUM only the filtered values not the whole population:
http://office.microsoft.com/en-gb/excel-help/subtotal-function-HP010062463.aspx
The reason why it is function_num 9 and not 109 is because filtered numbers are not hidden.
No but you could duplicate the sheet in another tab and have 1 displaying the paid total and the other the unpaid these values could then be displayed together on a 3rd sheet.
Ousethe VBA solution above assuming that it works.
You filter by text colour (right click on a filtered column for the options) the subtotal formula using 9 only sums the currently filtered for rows.
I can make a mock up and stick it on dropbox if you want.