Very interested in this as a feature, in fact I am was surprised to find that its not a standard feature in WM-Invoice.
A company that issues invoices should also be able to issue credit notes. Otherwise you wont be able to track and bookkepp returns or other credits issued to customers without having to resort to writng manual credit-notes.
Since a credit note is basically a negative invoice with its own numbering sequence, it should be a pretty simple thing to add but I wouldnt mind paying a reasonable fee for it, if it came to that.
I really enjoy working with WM-invoice and it solved a lot of the inherent problems in VM2 order handling for me, but its simply not a full invoicing solution if a credit note cant be issued from the backend, so you can keep track of credits issued in the cashflow.
Looking forward to seeing a patch. Keep up the good work!
Thanks!