Noptin_Abstract_Ecommerce_Integration::add_order_subscriber( int $order_id )

	public function add_order_subscriber( $order_id ) {

		// Fetch the subscriber id and order customer details.
		$subscriber_id      = $this->get_order_subscriber( $order_id );
		$subscriber_details = $this->get_order_customer_details( $order_id, empty( $subscriber_id ) );

		// Either create a new subscriber...
		if ( empty( $subscriber_id ) ) {

			// Should we process the subsriber?
			if ( ! $this->triggered( $order_id ) ) {
				return null;
			}

			return $this->add_subscriber( $subscriber_details, $order_id );
		}

		// Or update an existing one.
		return $this->update_subscriber( $subscriber_id, $subscriber_details, $order_id );

	}