Charting

I am trying to develop a chart that can be used as an
order form where the spreadsheet will POTENTIALLY
add "item a" item b" and "item c" to deliver a total. For
example:

4 pens @ 1.00 = 4.00
3 clips @ 50c = 1.50
6 pencils @ 40c = 2.40
TOTAL = 7.90

I've completed the chart to do this, but can not make #s
add and total unless all items are included. For example,
if I leave out the Clips it will read:

4 pens @ 1.00 = 4.00
!VALUE
6 pencils @ 40c = 2.40
TOTAL =

Question - how can I make this work if I do not have to
include all items on EVERY order?