Not ALL reps on the phone will allow a cancellation, so don't rely on that.

With mine, I cancelled the 1st order. The rep first said no, he can't and won't. Then he supposedly called the restaurant and cancelled it, said my $ would be refunded. So I placed a 2nd order, being more careful. Still no evidence of any code applied. Both orders received and both orders charged, no $ refunded.

SE only told me if I have issues, contact Grubhub, who is totally useless. Very disappointed.