You can do most of that (pretty simply) in excel.

For your data C2:C10 - Do a field: sum(C2:C11) Then when you want to add more data - right-click on the row (11) and "insert" That will update your sum calculation.

Also - you can do named fields - so that if you select the fields C2:C11 - then you can name them as "invoices" (in excel 2003 it's in the top left corner - there's a selection box you can type in. Just select and type a name in there). The lets you do the command sum(invoices)

Also - don't forget you can do something like sum(C:C) which will just give you everything in C column..