How to enable free shipping for particular quote even if it is disabled from admin?

Question

I want to enable/disable free shipping based on my custom quote conditions, it works fine when free shipping is enabled from admin but I want it to work also when free shipping is disabled from admin. I tried the following solutions but none of them worked for me - How do I enable free shipping in backend only (for order entry)

How to programatically disable shipping method in magento

https://stackoverflow.com/questions/26403751/how-to-get-enable-freeshipping-for-special-customers-in-magento-according-to-a

Answer 1

You need to rewrite the following class:

Mage_Shipping_Model_Shipping

and modify the following method to:

 /**
 * Get carrier by its code
 *
 * @param string $carrierCode
 * @param null|int $storeId
 * @return bool|Mage_Core_Model_Abstract
 */
public function getCarrierByCode($carrierCode, $storeId = null)
{
    if (($carrierCode == 'freeshipping' && $customCondition) || !Mage::getStoreConfigFlag('carriers/'.$carrierCode.'/'.$this->_availabilityConfigField, $storeId)) {
        return false;
    }
    $className = Mage::getStoreConfig('carriers/'.$carrierCode.'/model', $storeId);
    if (!$className) {
        return false;
    }
    $obj = Mage::getModel($className);
    if ($storeId) {
        $obj->setStore($storeId);
    }
    return $obj;
}

and you need to rewrite

Mage_Shipping_Model_Carrier_Freeshipping

(or better create a new carrier based on freeshipping)

and make the following changes:

/**
 * FreeShipping Rates Collector
 *
 * @param Mage_Shipping_Model_Rate_Request $request
 * @return Mage_Shipping_Model_Rate_Result
 */
public function collectRates(Mage_Shipping_Model_Rate_Request $request)
{
    if (!$customCondition && !$this->getConfigFlag('active')) {
        return false;
    }

    $result = Mage::getModel('shipping/rate_result');

    $this->_updateFreeMethodQuote($request);

    if (($request->getFreeShipping())
        || ($request->getBaseSubtotalInclTax() >=
            $this->getConfigData('free_shipping_subtotal'))
    ) {
        $method = Mage::getModel('shipping/rate_result_method');

        $method->setCarrier('freeshipping');
        $method->setCarrierTitle($this->getConfigData('title'));

        $method->setMethod('freeshipping');
        $method->setMethodTitle($this->getConfigData('name'));

        $method->setPrice('0.00');
        $method->setCost('0.00');

        $result->append($method);
    }

    return $result;
}