Add custom calculation to cart total and grand total in magento

Question

I am working on site where i want to add/subtract fee to cart total and grand total.I am firing this event to capture the cart details.sales_order_save_after. while in observer i got the price using this code

public function modifyPrice(Varien_Event_Observer $obs)
        {


       $getotal = Mage::helper('checkout')->getQuote()->getGrandTotal();
       $totals = Mage::getSingleton('checkout/session')->getQuote()->getTotals(); 
       $subtotal = $totals["subtotal"]->getValue();
}.

But i don't know how to add/subtract amount from total and than update accordingly.Thanks in advance

Answer 1

Rehman,

i am afraid this is not a straight forward task to complete, although you can take following as Ref to start

http://www.excellencemagentoblog.com/magento-add-fee-discount-order-total

Hope above link helps.

Answer 2

After searching i found this tutorial http://magento.ikantam.com/qa/how-add-discount-total-magento.And on Discount model class i can add/subtract custom price to cart total like :

 public function collect(Mage_Sales_Model_Quote_Address $address) {
            if ($address->getData('address_type') == 'billing')
                return $this;

                $discount = Mage::helper('my_module')->getCurrentdiscount(); // Custom percentage

                $grandTotal = $address->getGrandTotal();
                $baseGrandTotal = $address->getBaseGrandTotal();

                $totals = array_sum($address->getAllTotalAmounts());
                $baseTotals = array_sum($address->getAllBaseTotalAmounts());

                $address->setFeeAmount(-$totals * $discount / 100);
                $address->setBaseFeeAmount(-$baseTotals * $discount / 100);

                $address->setGrandTotal($grandTotal + $address->getFeeAmount());
                $address->setBaseGrandTotal($baseGrandTotal + $address->getBaseFeeAmount());
        return $this;
    }