Apple has introduced this "Double Click to Confirm" feature with the new iPhone X. If you double click on the long side button, then the Face ID will take over to confirm the purchase. Face ID needs to be set up in the Settings app under Face ID & Passcode. You can also disable it in the same place.
More information on how to use the new Face ID feature here (purchases are about halfway down the page) can be found here:
Otherwise, if you have any other questions, or if there is anything else I can do for you, please let us know!