You don't need to double-tap. Just touch (don't press) your thumb to the Touch ID sensor and put the back of the phone on the reader. The phone will wake up and

You don't need to double-tap. Just touch (don't press) your thumb to the Touch ID sensor and put the back of the phone on the reader. The phone will wake up and automatically pay with your default card, as long as your registered finger is on the sensor.