+=item gateway_callback_url - For ThirdPartyPayment only, set to the URL that
+the user should be redirected to on a successful payment. This will be sent
+as a transaction parameter (named "callback_url").
+
+=item gateway_cancel_url - For ThirdPartyPayment only, set to the URL that
+the user should be redirected to if they cancel the transaction. PayPal
+requires this; other gateways ignore it.
+