I have canceled under the 24 hour rule, agent said the ecert would be good....it was not. Call in again, agent said, "I'll get a supervisor to unlock it." Again no good....call in again, "once used and ticketed, they are deem used." It was an absolute no, even though before I canceled the ticket, I asked the question and received the wrong answer. Live and learn. Only time you can back out is if you just booked it and it has NOT been ticketed yet. Call reservation line immediately to have any chance to use it again.