public function add_meta_data( $key, $value, $unique = false ) {
		if ( $this->is_internal_meta_key( $key ) ) {
			$function = 'set_' . $key;

			if ( is_callable( array( $this, $function ) ) ) {
				return $this->{$function}( $value );
			}
		}

		$this->maybe_read_meta_data();
		if ( $unique ) {
			$this->delete_meta_data( $key );
		}
		$this->meta_data[] = new WC_Meta_Data(
			array(
				'key'   => $key,
				'value' => $value,
			)
		);
	}