Revising an Invoice
An invoice and its orders are locked against monetary changes in every status
except "Draft". To revise one that has already been opened, set its Status back
to "Draft", make your changes, then set it back to "Open".
You can do this from the invoice's Status field, or from the prompt shown on any
locked order.
Do not delete an invoice in order to change one of its orders. Reverting it to "Draft" does the same job and keeps the invoice number, its payments, orders and activity.
Revising versus voiding
Voiding is the destructive operation. It unlinks every order from the invoice. Propcart refuses to void or write off an invoice that has payments logged against it — clear the payments first if that is really what you want.
See Invoice Statuses for what each status allows.