'Perform PayPal PDT Postback validation.
Sends the transaction ID and business token to PayPal which responses with
SUCCESS or FAILED.'

def _postback(self):

    postback_dict = dict(cmd='_notify-synch', at=IDENTITY_TOKEN, tx=self.tx)
    postback_params = urlencode(postback_dict)
    return urllib2.urlopen(self.get_endpoint(), postback_params).read()
