public function has_items() {
		if ( null === $this->invoice_number ) {
			if ( ! $this->exists() ) {
				return false;
			}

			$invoice_number = $this->read_invoice_number();

			$transient = (array) get_transient( self::get_transient_name( $invoice_number ) );

			return count( array_filter( $transient ) );
		}

		return count( array_filter( $this->items ) );
	}