Add coupons codes to your WooCommerce emails using WP HTML Mail

functions.php

add_filter( 'haet_mail_placeholder_menu', 'add_mailbuilder_coupons_placeholder' );
add_filter( 'haet_mail_order_placeholders', 'populate_mailbuilder_coupons_placeholder', 10, 3 );
function add_mailbuilder_coupons_placeholder( $placeholder_menu ){
    if( is_array( $placeholder_menu ) ){
        $placeholder_menu[] = array(
                'text'      => 'Coupons',
                'tooltip'   => '[COUPONS]',
            );
    }
    return $placeholder_menu;
}

function populate_mailbuilder_coupons_placeholder( $order, $wc_order, $settings ){
    $coupons_text = '';
    if( $wc_order ){
        $coupons =  $wc_order->get_used_coupons();
        if( is_array( $coupons ) && count( $coupons ) ){
            $coupons_text .= 'You have used the following coupons: <br>';

            foreach( $coupons as $coupon_name ){

                // Retrieving the coupon ID
                $coupon_post_obj = get_page_by_title($coupon_name, OBJECT, 'shop_coupon');
                $coupon_id = $coupon_post_obj->ID;

                // Get an instance of WC_Coupon object in an array(necesary to use WC_Coupon methods)
                $coupons_obj = new WC_Coupon($coupon_id);
                $discount_type = $coupons_obj->get_discount_type();
                $amount_str = '';
                if( $discount_type == 'fixed_cart' )
                    $amount_str = '- €' . $coupons_obj->get_amount();
                elseif( $discount_type == 'percent' )
                    $amount_str = '- ' . $coupons_obj->get_amount() . '%';
                //elseif( $discount_type == ...
                
                $coupons_text .= '<strong>' . $coupons_obj->get_code() . '</strong>: ' . $amount_str . ', ';

            }
        }
    }
    $order['coupons'] = $coupons_text;
    return $order;
}