A "cancel purchase" button that is just 8pt text in the top corner of the screen is less Salient than a button with the same function that is the same size and style as the "confirm" button - i.e. in a readable size in a prominent place on the screen with an actual background to make it clear that it's a button.

Stuff that isn't salient doesn't stand out to the user. That makes them more likely to miss the option and decide to take a different one they otherwise might not have taken.