Cart price rule - apply free shipping for any method - php

Question

I have an extension to provide different rates for shipping, this module has got an option in the admin panel that let you choose the free shipping method if the cart price rule is met( if cart price rule requirement are respected, then set this method as free).
The problem is that I can choose only one method, but in the same country I offer 2 different carrier to delivery the package (lowest price for different regions) and this is a problem.
The extension save the value in the core_config_data table under path=carriers/premiumrate/free_method. I don't have clear how magento access this information so I have run these comand through SSH:
grep -rnw 'app' -e "carriers/premiumrate/free_method"
with no result, so I have tried:
grep -rnw 'app' -e "free_method"
Results:
app/code/core/Mage/Shipping/Helper/Data.php:159: $freeMethod = Mage::getStoreConfig('carriers/' . $arr[0] . '/free_method', $storeId);
app/code/core/Mage/Shipping/Model/Carrier/Abstract.php:56: protected $_freeMethod = 'free_method';
Next search:
grep -rnw 'app' -e "freeMethod"
Result:
app/code/core/Mage/Shipping/Helper/Data.php:159: $freeMethod = Mage::getStoreConfig('carriers/' . $arr[0] . '/free_method', $storeId);
app/code/core/Mage/Shipping/Helper/Data.php:160: return $freeMethod == $arr[1];
app/code/core/Mage/Shipping/Model/Carrier/Abstract.php:343: $freeMethod = $this->getConfigData($this->_freeMethod);
app/code/core/Mage/Shipping/Model/Carrier/Abstract.php:344: if (!$freeMethod) {
app/code/core/Mage/Shipping/Model/Carrier/Abstract.php:351: if ($item->getMethod() == $freeMethod) {
app/code/core/Mage/Shipping/Model/Carrier/Abstract.php:363: $this->_setFreeMethodRequest($freeMethod);
app/code/core/Mage/Shipping/Model/Carrier/Abstract.php:373: && $rate->getMethod() == $freeMethod
So I opened app/code/core/Mage/Shipping/Model/Carrier/Abstract.php and these are the lines (370-378; I have added the complete function at the bottom) that I have found interesting:
if (count($rates) > 1) {
foreach ($rates as $rate) {
if ($rate instanceof Mage_Shipping_Model_Rate_Result_Method
&& $rate->getMethod() == $freeMethod
) {
$price = $rate->getPrice();
}
}
}
Is it the correct place that I have to edit?
if I remove the if condition, will I destroy my site?
What should I do to set any shipping free if the cart rule is
active/valid?
Is there a better way to find out what I have to edit
instead of random grep?
Complete function:
protected function _updateFreeMethodQuote($request)
{
if ($request->getFreeMethodWeight() == $request->getPackageWeight() || !$request->hasFreeMethodWeight()) {
return;
}
$freeMethod = $this->getConfigData($this->_freeMethod);
if (!$freeMethod) {
return;
}
$freeRateId = false;
if (is_object($this->_result)) {
foreach ($this->_result->getAllRates() as $i=>$item) {
if ($item->getMethod() == $freeMethod) {
$freeRateId = $i;
break;
}
}
}
if ($freeRateId === false) {
return;
}
$price = null;
if ($request->getFreeMethodWeight() > 0) {
$this->_setFreeMethodRequest($freeMethod);
$result = $this->_getQuotes();
if ($result && ($rates = $result->getAllRates()) && count($rates)>0) {
if ((count($rates) == 1) && ($rates[0] instanceof Mage_Shipping_Model_Rate_Result_Method)) {
$price = $rates[0]->getPrice();
}
if (count($rates) > 1) {
foreach ($rates as $rate) {
if ($rate instanceof Mage_Shipping_Model_Rate_Result_Method
&& $rate->getMethod() == $freeMethod
) {
$price = $rate->getPrice();
}
}
}
}
} else {
/**
* if we can apply free shipping for all order we should force price
* to $0.00 for shipping with out sending second request to carrier
*/
$price = 0;
}
/**
* if we did not get our free shipping method in response we must use its old price
*/
if (!is_null($price)) {
$this->_result->getRateById($freeRateId)->setPrice($price);
}
}