dynamically add rows to SUM() calculation in excel

dynamically add rows to SUM() calculation in excel

I'm trying to set up an Excel spreadsheet in such a way that it can be
used to track orders over time and will let you see the total products
sold over time. Each time a new Purchase Order comes in, I want to be able
to add new row to the sheet, and have my total update without having to
manually change the formulae, letting me use the spreadsheet as a
database, ultimately adding buttons to add new orders, remove them,
calculate totals, etc.
For example, I would have a sheet like this:
order number qty of product sold
1 10
2 30
3 17
I want to then be able to add a new order to the end of this, and have the
total sales be calculated automatically including the new value. Ideally,
I'd like to avoid using too much VBA because some people using this
spreadsheet may only have access to OpenOffice, for which some of this may
not be compatible - if VBA is essential, then that's fine - I'd rather
work around access issues than decreasing usability of the spreadsheet for
everyone. links to tutorials that might be relevant would also be most
helpful - my searches so far have been having very limited success.
Thanks in advance for any suggestions you guys might have.