When you pre-order a song you don't pay for it until it's released and you download it. You've received the email informing you that it's available to download ? From http://support.apple.com/kb/HT5714 :
When the pre-order item becomes available, your account will be billed for the pre-order, and the item will appear in the Purchased section of your account. If there is a billing issue that prevents us from billing your account when the item is released, you'll receive an email notifying you that the item is available. You need to click the link within the email, using a computer and the latest version of iTunes, to fulfill the pre-order.
Download a pre-ordered item
When a pre-ordered item becomes available, you will receive an email letting you know that you can download your pre-order.
If you enabled the option to automatically download prepurchased content in iTunes, then your pre-ordered content may already be in your library. If not, click the download link provided in your email notification, and your pre-ordered content will begin to download. You can also use the Check for Available Downloads feature in iTunes on your computer (choose Store > Check for Available Downloads) to begin downloading your pre-order.