Hi Susan,
OK - you can't delete an invoice, so the trick is to find some way of making sure that an order that is shipped is immediately invoiced. How about creating a document that the shipping department has to print before an order goes out the door and making it an "invoice". If the shipping department prints this document for inclusion with the order, then Adagio will think an invoice has been printed and prevent people from deleting it.
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Andrew Bates