Double the Shipping Charges for certain items count at the cart

Question

I wish to modify the shipping cost Double if they add more than 5 items at the cart.

class ChangeShipping_MyCarrier_Model_Carrier extends
    Mage_Shipping_Model_Carrier_Abstract implements
    Mage_Shipping_Model_Carrier_Interface
{
    protected function _getStandardShippingRate()
    {
        $rate = Mage::getModel('shipping/rate_result_method');
        $rate->setCarrier($this->_code);
        $rate->setCarrierTitle($this->getConfigData('title'));
        $rate->setMethod('standand');
        $rate->setMethodTitle('Standard');

        $items_count = Mage::helper('checkout/cart')->getItemsCount();
        if ($items_count > 6) {
            $rate->setPrice(19.99);
        } else {
            $rate->setPrice(9.99);
        }
        $rate->setCost(0);
        return $rate;
    }
}

This function executes flawless without IF condition but shows error then.

Answer 1

In the carrier model please use $request object, and not the $cart object, as we can create the order from admin, API and other sources. See example here:

\app\code\core\Mage\Shipping\Model\Carrier\Flatrate.php

You need is to modify the collectRates based on you logic:

...
if ($request->getPackageQty() > 6) { // modify price }
...