+ 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;