Shake-to-pay just means that if you’re on another screen of the app when you get to the front of the queue, you can simply shake your iPhone to bring up the barcode instantly.
The tipping function allows you to adjust the amount of your tip for up to two hours afterwards, presumably in case they manage to spill your second cup of Americano or the barista creates a particularly artistic piece of latte art for you.
The current free Starbucks app, incidentally, is Sleep Cycle alarm clock, normally $0.99.
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