Modify item price in cart after placing order using sales_quote_add_item

We can easily modify product price after placing order in magento using the event sales_quote_add_item.

We can see this hook registered in app/code/core/Mage/Sales/Model/Quote.php line:874. This is the event created by magento.
Mage::dispatchEvent('sales_quote_add_item', array('quote_item' => $item));

We can access $item values using its registered event name sales_quote_add_item and we can modify the price with our logic.

Follow the steps:

1. Create a new module. Register our module in magento.
   Create a file in app/etc/modules/Company_All.xml
   

   
       <?xml version="1.0"?>
       <config>
         <modules>
           <Company_Product>
             <codePool>local</codePool>
             <active>true</active>
           </Company_Product>
         </modules>
       </config>
       

2. Create configuration file for our module file in app/code/local/Company/Product/etc/config.xml
   

   
       <?xml version="1.0"?>
       <config>
         <global>
           <models>
               <product>
                    <class>Company_Product_Model</class>
               </product>
           </models>
           <events>
             <sales_quote_add_item><!--Event to override price after adding product to cart-->
               <observers>
                 <company_product_price_observer><!--Any unique identifier name -->
                   <type>singleton</type>
                   <class>Company_Product_Model_Price_Observer</class><!--Our observer class name-->
                   <method>update_book_price</method><!--Method to be called from our observer class-->
                 </company_product_price_observer>
               </observers>
             </sales_quote_add_item>
           </events>
         </global>
       </config>
       

3. Create our observer file in app/code/local/Company/Product/Model/Price/Observer.php
   

   
       class Company_Product_Model_Price_Observer{
           public function update_book_price(Varien_Event_Observer $observer) {
               $quote_item = $observer->getQuoteItem();
               
               //if(){ //your logic goes here
                   $customprice = 50;
               //}
               $quote_item->setOriginalCustomPrice($customprice);
               $quote_item->save();
               return $this;
           }
           
       }
       

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