I have never had any difficulties refunding full fare tickets after check-in. It happens extremely frequently that I will book a shuttle flight for departure within the next 24 hours, check-in, print my BP only to have the meeting cancel or reschedule necessitating a refund. You simply need to call to have the check-in canceled and the ticket refunded. If you are at the airport when the cancellation occurs, you can also stop by the ticket counter or visit a club angel to have it refunded.