The item_number for each cart item in the IPN data is the eBay auction number. You can hit eBay's GetItemTransactions API to pull all of the transactions associated with that particular item/auction.
Of course, there could be more than one transaction associated with an individual auction, so to ensure you're grabbing the correct data you can use some XPath.
The eBay transaction data includes the PayPal transaction ID in what it calls "external transaction" data.
I just recently did all of this in a project of my own, and this is what I used to pull the specific data based on the PayPal transaction ID.
$DOM = new DOMDocument();
$DOM->loadXML($GetItemTransactionsResponse);
$XPath = new DOMXPath($DOM);
$XPath->registerNamespace("ns","urn:ebay:apis:eBLBaseComponents");
$eBayTransID = $XPath->query("//ns:Transaction[ns:ExternalTransaction/ns:ExternalTransactionID = '" . $txn_id . "']/ns:TransactionID/text()")->item(0)->data;
$shipping_method = $XPath->query("//ns:Transaction[ns:ExternalTransaction/ns:ExternalTransactionID = '" . $txn_id . "']/ns:ShippingServiceSelected/ns:ShippingService/text()")->item(0)->data;
$buyer_feedback_score = $XPath->query("//ns:Transaction[ns:ExternalTransaction/ns:ExternalTransactionID = '" . $txn_id . "']/ns:Buyer/ns:FeedbackScore/text()")->item(0)->data;
So the XPath being used there is basically saying "grab this data where the ExternalTransactionID = $txn_id, which is from the PayPal IPN data.
So it's a few pieces to plug together but it will work nicely for you.