Send coupons used in an order by email

wc-send-coupons-by-email.php

<?php

/**
 * Send an email each time an order with coupon(s) is completed
 *
 * The email contains coupon(s) used during checkout process
 *
 * @param int $order_id The ID of the order.
 */
function jp_email_order_coupons( $order_id ) {
	$order = new WC_Order( $order_id );

	if ( ! $order->get_used_coupons() ) {
		return;
	}

	// Load the order details using the WC hooks.
	ob_start();
	WC_Emails::instance()->order_details( $order );
	$order_details = ob_get_clean();

	$to      = 'youremail@yourcompany.com';
	$subject = 'New Order Completed';
	$headers = 'From: My Name <youremail@yourcompany.com>' . "\r\n";

	$message = 'A new order has been completed.\n';
	$message .= 'Order ID: ' . $order_id . '\n';
	$message .= 'Coupons used:\n';

	foreach ( $order->get_used_coupons() as $coupon ) {
		$message .= $coupon . '\n';
	}

	$message .= 'Order details: ' . $order_details . '\n';

	@wp_mail( $to, $subject, $message, $headers );
}

add_action( 'woocommerce_thankyou', 'jp_email_order_coupons' );