public function add_item( $item ) {
		$items_key = $this->get_items_key( $item );

		if ( ! $items_key ) {
			return false;
		}

		// Make sure existing items are loaded so we can append this new one.
		if ( ! isset( $this->items[ $items_key ] ) ) {
			$this->items[ $items_key ] = $this->get_items( $item->get_type() );
		}

		// Set parent.
		$item->set_order_id( $this->get_id() );

		// Append new row with generated temporary ID.
		$item_id = $item->get_id();

		if ( $item_id ) {
			$this->items[ $items_key ][ $item_id ] = $item;
		} else {
			$this->items[ $items_key ][ 'new:' . $items_key . count( $this->items[ $items_key ] ) ] = $item;
		}
	}