just get the id from the combobox of selection through request.form for example and then throw into the sql statement to update or insert say something like:

update AmountTable
set ValueColumn = (select AmountColumn from FruitTable where PrimaryId = 'SelectedValue' ) + (select ValueColumn from AmountTable where PrimaryId = 'SelectedValue')
where PrimaryId = 'SelectedValue'

you need to be more specific if you want more specific answer