as a general rule in crystal do not confuse getting data to display a particlular way (unduplicated) and actually altering your data set. The sppressed data is still part of the report and must be accounted for in any calculations.
lockwelle gave you the shared variable solution, here is a Running Total (RT) solution.
In field explorer find RTs and select New
Name=whatever
Field to summarize = INVOICE.CARR_NET
type of sumamry = SUM
evaluate = on change of field (INVOICE.ORDER_NUMBER)
reset= never
place in report footer
you can alter the reset location if you have groups and need sumas at some other group leve
you can also alter the selection in evaluate to use on change of group if your group structure makes sense for it.