I preordered an album on iTunes and it won't let me download it. It's not in my purchased or anything and when it says I have purchased all the songs. I have one single from the album and when I try to "Conplete my album" it charges me for it

Hello TheCoddfish,

Thank you for using Apple Support Communities!

So it sounds like you have a preorder you would like to download, but it is not downloading, or you dont have the option to download it.

Without knowing what the preorder was, and if it is released, I found this article named iTunes Store: About Pre-orders found here http://support.apple.com/kb/HT5714.

This part in particular:

Downloading a pre-ordered item

When a pre-order item becomes available, you will receive an email letting you know your pre-order is available for download.

If you've enabled iTunes to automatically download prepurchased content, then your pre-order content may already be downloaded to your library. If not, simply click the download link provided within your email notification, and your pre-order content will begin to download. Alternatively you can use the "Check for Available Downloads" feature in iTunes on your computer by choosing Store > Check for Available Downloads to begin downloading your pre-order.

To view, cancel, or see the expected release dates for your pending pre-orders

1. Choose Store > View My Appie ID, then click Manage Pre-orders, which located in the Settings section at the bottom of the Account Information screen in iTunes on your computer.
   
2. From the Mange Pre-orders screen, click the Cancel button to the right of the pre-order item you would like to cancel. You'll be asked to confirm your pre-order cancellation.
   

If the album is released and you still cannot download it, I would report the issue directly to the iTunes Store support.

How to report an issue with your iTunes Store, App Store, Mac App Store, or iBooks Store purchase

http://support.apple.com/kb/ht1933

All the best,

Sterling

Same happened to me when trying to pre order MMLP2. Only took 5 minutes to figure out. On your itunes store at the top of the screen where it says your username, or email for apple ID. Click it and your account information will pop up. One of the options was saying "you have 18 songs ready for download". Click that and they will start downloading. Easy fix, though auto download the second the album is released would make more sense.