You didn't owe iTunes any money, you've checked the purchase history on your account and you didn't have any pre-orders and/or auto-renewing subscriptions that were due ?
Viewing purchase history : http://support.apple.com/kb/HT2727
If not then have you tried logging out and back into your account on your iPod or computer and seeing if your balance then shows the correct amount ? If it doesn't then try contacting iTunes Support and see if they can tell you where the $8 went : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption