Cancel specified letter
        :param pixelletter_id: ID of the letter
        :return: True if canceled, else False

def cancel_letter(self, pixelletter_id):
        """
        Cancel specified letter
        :param pixelletter_id: ID of the letter
        :return: True if canceled, else False
        """
        cancel_request = self._make_delete_request('letters/{}'.format(pixelletter_id))
        status = json.loads(cancel_request)['status']

        if status == 200:
            return True

        return False