Cash On Delivery and Fees

Hi all,
I'm having some issues with COD and cod fee modules:

1. Cash On Delivery always show as first selection even if order is 999
   Note that I've only paypal and cod enabled (and paypal is set to 0)

2. Cod is available only to a certain country and with some shipping method (not all)
   I added some code to cod.php that disables it when the shipping method selected isn't right.

So the whole update_status function looks like :

// class methods
    function update_status() {
      global $order, $db;

      if ( ($this->enabled == true) && ((int)MODULE_PAYMENT_COD_ZONE > 0) ) {
        $check_flag = false;
        if (IS_ADMIN_FLAG === false) {
        $check = $db->Execute("select zone_id from " . TABLE_ZONES_TO_GEO_ZONES . " where geo_zone_id = '" . MODULE_PAYMENT_COD_ZONE . "' and zone_country_id = '" . $order->delivery['country']['id'] . "' order by zone_id");
        } else {
        $check = $db->Execute("select zone_id from " . TABLE_ZONES_TO_GEO_ZONES . " where geo_zone_id = '" . MODULE_PAYMENT_COD_ZONE . "' order by zone_id");
        }
        while (!$check->EOF) {
          if ($check->fields['zone_id'] < 1) {
            $check_flag = true;
            break;
          } elseif ($check->fields['zone_id'] == $order->delivery['zone_id']) {
            $check_flag = true;
            break;
          }
          $check->MoveNext();
        }

        if (! ( ($order->info['shipping_module_code'] == 'celere3_celere3') || ($order->info['shipping_module_code'] == 'celere1plus_celere1plus') ) ) {
                $this->enabled = false;
                $check_flag = false;
                /* COD available only on poste Italiane (UGLY!!!) */
        }

        if ($check_flag == false) {
          $this->enabled = false;
        }
      }

But it happens (not always) that a customer selecting courier (and then not celere3 or celere1plus is presented to pay the cod_fee
shouldn't cod be disabled by my change if a customer don't select the needed shipping modules ?

3. where I've to write the piece of code that update the checkout_payment page when the user change the selected payment ? I'd like to show the differences in the price to customer without having them to reload the page

I mean if I select paypal as payment there won't be the cod fee and so the order total will be less than when selecting COD.
I want to update (via javascript/DOM) the order total (and the lines) without having the customer to submit the form.