public static function fix_mp3_file_url($value, $post_id, $raw, $field)
        {
            return self::verify_meta($value, $post_id, 'file_url');
        }
        public static function fix_mp3_on_sale($on_sale, $product)
        {
            $course_id = empty(self::$course_id) ? get_post_meta($product->ID, 'course_id', true) : self::$course_id;
            if (!empty($course_id)) {
                $on_sale = (int) get_post_meta($course_id, 'mp_is_sale', true);
            }
            return $on_sale;
        }
    }
    /**
     * For Reference:  MP2.0 vs MP 3.0
     * ===============================
     *    mp_sku             => sku
     *    mp_price           => regular_price
     *    mp_sale_price      => sale_price_amount
     *    mp_track_inventory => track_inventory
     *    mp_inventory       => inventory
     *    mp_special_tax     => special_tax_rate
     *    mp_is_sale         => has_sale
     *    mp_shipping        => extra_shipping_cost
     *    mp_shipping        => weight_extra_shipping_cost
     *    mp_file            => file_url
     *    mp_product_link    => external_url
     */
}
CoursePress_MarketPress_Integration::init();