+  return 'invoice_for_advance_payment_storno' if $self->type eq 'invoice_for_advance_payment' && $self->amount < 0 &&  $self->storno;
+  return 'invoice_for_advance_payment'        if $self->type eq 'invoice_for_advance_payment';
+  return 'final_invoice'                      if $self->type eq 'final_invoice';
+  # stornoed credit_notes are still credit notes and not invoices
+  return 'credit_note'        if $self->type eq 'credit_note' && $self->amount < 0;