My lease included a 60 day notice. I gave a 30 day notice and moved out at the end of my lease. I was billed for the additional 30 days. After moving out, I found out that my apartment was re-rented within 8 days. I asked for an updated statement covering the amount of days the apartment was vacant. The leasing office states that the 60 notice is a fee not rent and can 't be adjusted/credited. Is this legal?

There is no "rule" or "law" this is merely the terms of the contract you signed.

What is the wording ? Is it a fee or is it rental due to short notice?

Does not sound like a fee to me.

But they may bill you (incorrectly) they may turn it over to collections, even try to put it on your credit report.

One option almost no one does, pay them, and then sue them for the damages.

As Chuck said, this is in the terms of your lease, its not an issue of law. You agreed to give 60 days notice.

You can try to sue them in small claims court. Most areas do not permit double dipping.