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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _postback(self):
 """ Perform PayPal PDT Postback validation. Sends the transaction ID and business token to PayPal which responses with SUCCESS or FAILED. """

        return requests.post(self.get_endpoint(),
                             data=dict(cmd="_notify-synch", at=IDENTITY_TOKEN, tx=self.tx)).content