send-coupons-used-in-an-order-by-email.php

/**
 * Send an email each time an order with coupon(s) is completed.
 * The email contains coupon(s) used during checkout process.
 */
function wb_woo_email_order_coupons( $order_id ) {
	$order = new WC_Order( $order_id );

	if ( $order->get_used_coupons() ) {

		$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';
		}
		wp_mail( $to, $subject, $message, $headers );
	}
}
add_action( 'woocommerce_thankyou', 'wb_woo_email_order_coupons' );