Have you been looking for a cost-effective way to fill up your new iPad or iPhone 4S with music and apps? Best Buy has you covered, offering 20 percent off iTunes gift cards when purchased online. The cards are available in $15, $25, $50, and $100 denominations, but you'll be taking them home from between $12 to $80 — and digital delivery means no shipping costs, either. The cards are good for any purchases made through iTunes, including Apple TV movie and TV rentals; they won't work in your local Apple Store, unfortunately. It's not clear how long the sale will be going on, but these discounts can vanish as quickly as they appear, so we suggest you act quickly!