The new Apple Store app allows you to adjust your purchases yourself in Apple stores

Apple
works a lot on improving its physical shops and improves
regularly the services offered. Latest, a small
revolution since the new version (2.0)
Apple Store Apple applicationintroduces several new features including the
possibility ofPay your purchases in the Apple StoreOne would
without going through a seller.
CalledEasyPayThis functionality is not currently
available only in the USA and limited to a few products but it is doomed to
deploy more widely let's be sure:

Recently announced, the new Apple Store application is
available.
Happy American users will be able to enjoy easypay if they want
Buy certain products more quickly such as accessoriesin
crowdIn their favorite Apple Store.
Nevertheless the most expensive Apple products (MacBook, iPhone, iPod Touch) being
keyin the stockstores, it will always be necessary
Contact a seller to buy them.
The principle issimple:scannez(with
the camera of the iPhone 4 or 4S) the product barcode you want
buy, access information relating to it (notes,
specifications, etc, ...) andpay offwith the credit card
referenced in your Appleid/iTunes account.

Other new products:
What’s New in Version 2.0

• "Personal pickup": buy via the application and choose the Apple Store in
  whichwithdraw your purchase, usually an hour later
  order. If the delay is more important you will be warned of its availability
  in store (USA only),
• "Track your stuff":follow uporder (available in
  France),
• Support for Canadian and Chinese stores.

Would you like to see Easypay arrive quickly in Europe? I would also like
be able to pay with the money loaded by iTunes card, not only with a
Credit card, not you?
L’applicationApple Storeis available for free
Here on the AppStore.
Source