[TYPO3-commerce] Basket Sum gross not reflected back in paypal

[Siddhesh]

Hello List,
  Let me first explain my problem.I have to calculate the sum gross with 
some additional cost calculated depending on weight on the checkout page.
So what i did was
1)$sum_gross=$baskoj->get_gross_sum(true);

//I have used the hook 
$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['commerce/lib/class.tx_commerce_pibase.php']['makeBasketInformation'][]='EXT:commerce_basket/pi1/class.tx_commercebasket_pi1.php:&tx_commercebasket_pi1';

2)$sum_gross=$sum_gross+$firstclassprice;
3)$markerArray['###SUM_GROSS###']=$sum_gross;

Now after checkout it goes to paypal.But on paypal i get the old amount 
not the new one.

Can u one help me out whivh hook shoul i use.

Thanks in Advance