Re-Purchasing iTunes Stuff after Wipe.

I recently re-installed my OS, and lost everything (I wanted it that way)
Now I want to re-download all my music, and in iTunes if you have already purchased something you can just redownload it and it wont charge you...
BUT IT CHARGED ME! It was only 7.99, but i have about 100$ of other stuff I need to re-download.. Why did it charge me!? And I hit "Check for available downloads" but it didn't show anything...

I'd suggest contacting Apple support direct as, according to the article linked below, you cannot re-download songs, videos, iPod Games and albums.

iOS apps are the exception, so long as you long in with the same iTunes account they were purchased on they can be re-download.

Hopefully this article is out of date iTunes Store: Purchased songs, videos, iPod Games, and albums can be downloaded only once

But I'd definately call them and see if they can put an exception on your account.

That article is not out of date. You can download music and videos only one time. You need to take care of backing up your own data.

Not sure what you thought it would do when you hit that button that said "Buy".
Shouldn't be a problem though, just restore them all from your Time Machine backup.

Books purchased through iBooks can also be re-downloaded along with apps.

again, a good reason to use backup.