Order tracking information on completed email

add_action( 'woocommerce_email_order_details', function( $order, $sent_to_admin, $plain_text, $email ) {

	// Only For Specific Emails
	if(
		empty( $email->id )
		|| ! in_array( $email->id, [ 'customer_completed_order', 'customer_invoice' ] )
	) {
		return;
	}

	// Get ShipStation Status
	global $wpdb;
	$order_id = $order->get_id();
	$sql = "
		SELECT comment_content
		FROM {$wpdb->comments}
		WHERE comment_post_ID = %d
		AND comment_content LIKE '%% shipped via %%'
		ORDER BY comment_ID DESC
		LIMIT 1
	";
	$order_note = $wpdb->get_var( $wpdb->prepare( $sql, $order_id ) );

	// Handle No Results
	if( ! $order_note ) {
		return;
	}

	// Get Tracking Number From String
	$words = preg_replace( "/[^A-Za-z0-9 ]/", '', $order_note );
	$words = explode( ' ', $words );
	$last_index = sizeof( $words ) - 1;
	$tracking_num = $words[$last_index];

	// Detect USPS
	if( stristr( $order_note, 'USPS' ) ) {
		$order_note = str_replace( 'USPS', '<strong>USPS</strong>', $order_note );
		$tracking_link = 'https://tools.usps.com/go/TrackConfirmAction?qtc_tLabels1';
	}

	// Detect UPS
	else if( stristr( $order_note, 'UPS' ) ) {
		$order_note = str_replace( 'UPS', '<strong>UPS</strong>', $order_note );
		$tracking_link = 'https://www.ups.com/track?loc=en_US&tracknum';
	}

	// Detect FedEx
	else if( stristr( $order_note, 'FedEx' ) ) {
		$order_note = str_replace( 'FedEx', '<strong>FedEx</strong>', $order_note );
		$tracking_link = 'https://www.fedex.com/fedextrack/?trknbr';
	}

	// Add Tracking Link
	if( ! empty( $tracking_link ) ) {
		$tracking_link = sprintf(
			'<a target="_blank" href="%s=%s">%s</a>',
			$tracking_link,
			$tracking_num,
			$tracking_num
		);
		$order_note = str_replace( $tracking_num, $tracking_link, $order_note );
	}

	// Output
	echo wpautop( $order_note );

}, 10, 4 );