I have lived in my apt complex going on 5 years. Last month I was late paying my rent a couple of days because I was out of town on business. When I got back and called to get the total amount due, it was an outrageously high amount of money. When I asked him to break down the charges, he said the initial $100 late fee plus $10 per day PLUS the concession fee. I had no clue what a "concession fee" was and he said "well its in your lease", they've never chosen to exercise that option before but that they put a note on everyone's door in April letting them know that in the future, if payments were received late, they would be charged the automatic $100 late fee, whatever concession fee is indicated in their lease PLUS $10 a day until it is paid. It was like 1/3 of my rent! I argued with him and fortunately he let it go that time but seriously... can they do that? It seems like they are double penalizing someone for being late.

I know things need to be paid on time. But there are times that people can't because of family emergencies or whatever. But charging so much doesn't help anyone except the landlord bring in more money.

Thanks.

Doesn't help anyone except the landlord bring in more money -exactly!
They can do whatever they want as long as it is written in the lease or they notified you of the change. Be thankful that he let you go this time. Maybe if you know you are going to be out of town and have no way of paying in advance or on time you could talk to your landlord about it now and ask him if you can give him a week or two notice asking to let you go on the late charges until a specific date when necessary. If he agrees get it in writing (even written in on your lease) and save copies of all notices you send him.

If you have a current written lease that is in effect, they can not change any item in your current lease. So if your current lease names the exact fee, it can not be changed, but if it merely ways they can set such fees, and does not name it, then yes they can change it