There is certainly no fee on top of the charge for the night. Where there is an "early check-out fee", it's to make up for the revenue lost by not charging you for the night that is missed. Since you paid with points and it's the free night, I can't see how there would be a fee.

If you need the night credit, just leave - they'll check you out the next day. There's really no need to check out online.

If you don't need the night, you could tell them you're leaving so they can use the room in an emergency. 6pm is probably too late to resell, but they may have a late walk-in or an oversell.