code: def generate_cart_upload_redirect_url(self, **kwargs):
          
          required_vals = ('business', 'item_name_1', 'amount_1', 'quantity_1')
          self._check_required(required_vals, **kwargs)
          url = "%s?cmd=_cart&upload=1" % self.config.PAYPAL_URL_BASE
          additional = self._encode_utf8(**kwargs)
          additional = urlencode(additional)
          return url + "&" + additional
docs: https://www.sandbox.paypal.com/webscr
              ?cmd=_cart
              &upload=1