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

			$invoice_number = $this->read_invoice_number();

			$items = (array) get_transient( self::get_transient_name( $invoice_number ) );
		} else {
			$items = $this->items;
		}

		return ! empty( $items[ $item_id ] ) ? (int) $items[ $item_id ] : false;
	}