sub items or sub totals

sub items or sub totals

i need to make invoices that show detailed list of items but not individual prices
how could i make an item that would group other items, this parent item having as rate the sum of the sub tiems rates ?
alternatively without creating a parent items i would like to have a subtotal that would hide the single items indivdual prices ?
eg

 eggs                  4
butter                  250 g
flour                       250 g
sugar                  250 g

subtotal ( or parent item) 
 cake                         1 at 5.49 = 5.49$

the 5.49 beeing comuted from the subitems prices