Well, you could put it on hold via telephone, and then still purchase online tomorrow as a new reservation if the fare is still there. (I recommend calling and cancelling the held reservation if you do book a new one online later.)

Or you could put it on hold, and gladly pay the $5 telephone booking fee tomorrow if the fare disappears.

Or you could buy online now and cancel within 24 hours for a refund if it turns out that it does not work for you.

BillJ