Surely every invoice (no matter how its created) needs to have 2 reference numbers allocated to it - the customers reference, and our reference.
Our reference is normally the invoice number
Their reference is the PO number or a 2-3 word description of the job
I cant see a way to allocate a customer reference/PO number to an invoice without opening a project. Id like PO numbers or Customer references to be able to be allocated to an invoice without being tied to a project