I believe this has been discussed before - if you have made changes to the reservation (I.e., booked a lower fare) the $5.60 goes to your travel funds.
Its best to cancel and rebook if you find a lower fare.
So I did this. I cancelled and rebooked after finding a lower fare. Was offered a refund back to original cc (and it posted).
Then, the fare dropped again. So I went to cancel (and rebook) and they only allowed travel credits this time (even though reservation was newly rebooked). Weird and annoying - but it was only $15 so i took the travel credit. Anyone know why this happened though? Are we only allowed to do this cancel/rebook once per?