Add the Coupon Code names to Woocommerce View Order details and email notifications

Updated - Handling discounts with a zero value

The following code will after "discount" line in order totals lines, displaying the applied coupons to the order:

add_filter( 'woocommerce_get_order_item_totals', 'add_coupons_codes_line_to_order_totals_lines', 10, 3 );function add_coupons_codes_line_to_order_totals_lines( $total_rows, $order, $tax_display ) {    // Exit if there is no coupons applied    if( sizeof( $order->get_used_coupons() ) == 0 )        return $total_rows;    $new_total_rows = []; // Initializing    foreach($total_rows as $key => $total ){        $new_total_rows[$key] = $total;        if( $key == 'discount' ){            // Get applied coupons            $applied_coupons = $order->get_used_coupons();            // Insert applied coupon codes in total lines after discount line            $new_total_rows['coupon_codes'] = array(                'label' => __('Applied coupons:', 'woocommerce'),                'value' => implode( ', ', $applied_coupons ),            );        }    }    return $new_total_rows;}

Display on customer order view, with 2 applied coupons:

Additional code version: Handle applied coupons with a zero discount amount use this instead:

add_filter( 'woocommerce_get_order_item_totals', 'add_coupons_codes_line_to_order_totals_lines', 10, 3 );function add_coupons_codes_line_to_order_totals_lines( $total_rows, $order, $tax_display ) {    $has_used_coupons = sizeof( $order->get_used_coupons() ) > 0 ? true : false;    // Exit if there is no coupons applied    if( ! $has_used_coupons )        return $total_rows;    $new_total_rows  = []; // Initializing    $applied_coupons = $order->get_used_coupons(); // Get applied coupons    foreach($total_rows as $key => $total ){        $new_total_rows[$key] = $total;        // Adding the discount line for orders with applied coupons and zero discount amount        if( ! isset($total_rows['discount']) && $key === 'shipping' ) {            $new_total_rows['discount'] = array(                'label' => __( 'Discount:', 'woocommerce' ),                'value'    => wc_price(0),            );        }        // Adding applied coupon codes line        if( $key === 'discount' || isset($new_total_rows['discount']) ){            // Get applied coupons            $applied_coupons = $order->get_used_coupons();            // Insert applied coupon codes in total lines after discount line            $new_total_rows['coupon_codes'] = array(                'label' => __('Applied coupons:', 'woocommerce'),                'value' => implode( ', ', $applied_coupons ),            );        }    }    return $new_total_rows;}

Display on email notifications with a coupon that has 0 discount:

Code goes in function.php file of your active child theme (or active theme). Tested and works.