/**
  * Set shipping charges on a country by country basis.
  * For example, SimpleShippingModifier::set_charges_for_countries(array(
  *   'US' => 10,
  *   'NZ' => 5,
  * ));
  * @param countryMap A map of 2-letter country codes
  */
 static function set_charges_for_countries($countryMap)
 {
     self::$charges_by_country = array_merge(self::$charges_by_country, $countryMap);
 }