Google Play Store supports fingerprint authentication for purchases on Marshmallow

The support for fingerprint authorization comes as a part of the new Play Store that is now rolling out with the snazzy redesign, but for whatever reason, Google doesn't have the option turned on by default on devices that have fingerprint scanners. If you have a device with a fingerprint scanner that is running Android 6.0 (which right now means you have a Nexus 5X or a review unit 6P), you'll have to dig into the Play Store Settings and look under User Controls in order to turn on fingerprint authentication. 

This should prove to make authorizing purchases in the Play Store much easier, but of course that also comes with the downside of potentially spending a lot more money in the Play Store than you might want. If you're worried about your spending habits, you may want to leave on the password entry, so that while you're typing in your password you can have a quiet moment to reflect on if you really need whatever it is you're buying. 

source: Android Police