This function/filter will add ics files from bookings created with WooCommerce Bookings to the Processing and Completed emails sent from WooCommerce itself.

jp_add_ics_to_woocommerce_emails.php

<?php // do not copy this line

/**
 * This function/filter will add ics files from bookings created with WooCommerce Bookings to 
 * the Processing and Completed emails sent from WooCommerce itself. 
 * @param  arr   $attachments Current array of attachments being filtered. 
 * @param  str   $email_id    The id of the email being sent.
 * @param  obj   $order       The order for which the email is being sent. 
 * @return arr                The filtered list of attachments. 
 */
function jp_add_ics_to_woocommerce_emails( $attachments, $email_id, $order ) {
	
	// The woocommerce email ids for which you want to attach the ics files.
	$available = array(
		'customer_processing_order',
		'customer_completed_order',
	);

	// Check to make sure we have a match. 
	if ( in_array( $email_id, $available ) ) {

		// Get the booking ids from the order, and get the exporter object.
		$booking_ids = WC_Booking_Data_Store::get_booking_ids_from_order_id( $order->get_id() );
		$generate    = new WC_Bookings_ICS_Exporter;

		// Go through each id and add the attachments.
		foreach ( $booking_ids as $booking_id ) {
			$booking       = get_wc_booking( $booking_id );
			$attachments[] = $generate->get_booking_ics( $booking );

			// If the object is not unset, then the New Booking Email is sent twice. 
			unset( $booking );
		}
	}

	return $attachments;
}
add_filter( 'woocommerce_email_attachments', 'jp_add_ics_to_woocommerce_emails', 10, 3 );