It took Apple a while to implement these what seem like no-brainer features for customers to check themselves out while purchasing accessories at Apple retail stores and paying via their iTunes account without having to wait for an employee to ring you up on an accessory on a busy day. Which just so happens to be almost every single business day. Apple has issued an update that enables you to use the Apple Store app with your iPhone 4 or iPhone 4S to make in-store purchases by scanning the barcode on the packaging without even having to interact with employees called EasyPay.
For larger purchases like Macs, iPods, iPads and iPhones you'll still need assistance of an employee. But if you don't feel like waiting, you can now purchase those devices beforehand and within an hour, pick up your purchase at your local Apple store without having to wait. That is until everyone else finds out about this. So have fun and shop responsibly!