Which US Apple stores accept iTunes Gift Cards? From time to time, retailers will discount iTunes Store Gift Cards and I'm tempted to stock up since discounts are enticing ways to shop. It appears that Apple has three sorts of gift cards available in the US. I am interested in the usually colorful or music related one.  

Originally, these cards only worked to buy music from the iTunes Store, but now my Apple ID works with several other store fronts in addition to the expected iTunes music store on iOS, Mac and Windows:


iBookstore on iOS
iCloud to buy additional storage space annually
iTunes match annual subscription service
App Store on iOS
App Store on Mac OS
Apple Store app on iOS for ordering product online or for pickup in Apple Retail Stores.
Apple Store on the internet at http://store.apple.com


Which of these will accept a US purchased credit from a physical or electronic iTunes Store Gift Card if your Apple ID has a US billing address? Which of these will not be able to spend credit redeemed from the iTunes gift card. I do know that the Apple Store gift cards are accepted at the Apple Retail stores, but I've never seen them discounted or available anywhere but from Apple Retail.
You can use redeemed value from an iTunes gift card to buy from the following stores:


iTunes music, movie and TV stores for iOS, Windows and Mac
iBookstore on iOS
App Store on iOS
App Store on Mac OS




The only gift cards that are accepted at an Apple Retail Store are Apple gift cards. The Apple Retail Stores and the Apple Store online do not accept iTunes gift cards as forms of payment. 

You can not use redeemed value from an iTunes gift card to buy an Apple gift card or anything else from the following stores:


Apple Retail Stores
Apple Store on the internet at http://store.apple.com
iCloud yearly storage space increases
iTunes Match service