Alright, for anyone unable to download a single after preodering an album, I have the solution (just tried it).
1. You need to temporarily cancel your preorder. On the top menu bar, click, Store, then View Account.
2. Scroll down to the bottom of your iTunes account page and click Manage Preorders.
3. Click the button to cancel to your preorder.
4. Go back to the album page and buy and download the single. The preorder price should drop since you bought the single.
5. Re-preorder the album, should be at the lower price. Now you have the single and have preordered the album, with no extra money spent.