Good write-up, but I’m not sure if I agree on your Starbucks example. The ‘PAY’ button is small and not somewhere I’d look first and it doesn’t make itself clearly visible because on most devices the black bar would be on top of a navigation bar, which means it won’t draw my attention.

And clicking on the card? While clever, that would not come to my mind at all to pay, without some kind of tooltip I probably wouldn’t even think of the possibility of paying that way. There is a lot of space underneath the card, why not add the button there?