Yes you can and no fee. Just did this last month and down to the wire.

Boarded aircraft and settled in before coming to realization I left an item at security. I knew there was no way to retrieve it in time and if I got off, I’m not making to destination that day since only one flight per day.

Gate agent came aboard with manifest and told FA boarding completed and I told her I’m not flying. Can’t get any more last minute than that.

She sent me up the jet bridge to speak with other gate agent. Other agent reopened the flight to offload me. I confirmed ticket status is OPEN via refunds page. Ticket also appear in wallet.