Magento 1.8 – Fix Mage::getSingleton(‘checkout/session’)->getLastRealOrderId() Issue

the download link for the product is not showing

This could mean the link as a whole does not show up, the link target is empty or it is not generated at all (no link_hash value) in the database.

The link to the downloadable Product gets generated within the sales_order_item_save_commit_after event in the saveDownloadableOrderItem method in Mage_Downloadable_Model_Observer.

So if it is not generated at all, have a look in your downloadable_link_purchased_item database table and check if there are values under link_hash and link_url or link_file fields.

If not, you might have another problem, with your e-mail template maybe.

Sometimes Paypal requests are wrong, so server don't fire Magento routes as it should and you get server error 500. For checking, is it your situation or not, try to do next:

1) Create folder named 'paypal' in Magento root folder.
2) Create file named 'ipn.php' in this 'paypal' folder.
3) Open file and add next content:

<?php
    date_default_timezone_set('America/Los_Angeles');
    require_once '../app/Mage.php';
    umask(0);
    $app = Mage::app('default');
    if ($app->getRequest()->isPost()) {
       return;
    }
    try {
        $data = $app->getRequest()->getPost();
        Mage::getModel('paypal/ipn')->processIpnRequest($data, new Varien_Http_Adapter_Curl());
    } catch (Exception $e) {
        Mage::logException($e);
        $app->getResponse()->setHttpResponseCode(500);
    } 
?>

4) Clear Magento cache and check is Paypal IPN start working for you.

If it start working, just keep it as is. When Paypal requests are correct - it will work with standard Magento way, otherwise - paypal/ipn.php file will be used.