Send email when a customer adds an item to their WooCommerce Subscription with Toolbox for WC Subscriptions plugin.

functions.php

<?php
/**
 * Send an email notification when a customer adds an item to their subscription.
 *
 * @param WC_Subscription $subscription
 * @param WC_Product $product
 * @param int $quantity
 */
function sp_toolbox_notify_add( $subscription, $product, $quantity ) {
	$to = ''; // recipient of the email
	$subject = 'Customer Added Item to Subscription'; // subject
	$body = 'A customer added a product to their subscription.' . '<br/>' . PHP_EOL;
	$body .= 'Subscription ID: ' . $subscription->get_id() . '<br/>' . PHP_EOL; // Subscription ID
	$body .= 'for customer: ' . $subscription->get_billing_first_name() . ' ' . $subscription->get_billing_last_name() . '<br/>' . PHP_EOL; // Customer Name
	$body .= 'Product: ' . $product->get_name() . '(' .  $product->get_sku() . ')<br/>' . PHP_EOL; // The product Name
	$body .= 'Quantity: ' . $quantity . '<br/>' . PHP_EOL; // The quantity
	$headers = array('Content-Type: text/html; charset=UTF-8');

	wp_mail($to, $subject, $body, $headers);
}
add_action('jgtb_added_item_to_subscription', 'sp_toolbox_notify_add', 10, 3 );