Paypal Express Checkout Return URL Customization in Magento

Solved it!

Uncommented “ini_set('display_errors', 1);” from my index.php page which enabled me to see an error message instead of the blank page. Turns out the problem was a conflict with our unirgy gift certificate module. After disabling that module everything works.

I spent 2 days but still could not find out who changed the value in AMT.

The following hack works for now. In app/code/core/Mage/Paypal/Model/Api/Nvp.php, change the method callDoExpressCheckoutPayment():

 ...
 $this->_exportLineItems($request);
 $finalTotal = $request['ITEMAMT'] + $request['SHIPPINGAMT'];
 if (sprintf('%.2F', $request['AMT']) != sprintf('%.2F', $finalTotal)) {
     $request['AMT'] = $finalTotal;
 }