You can purchase a Playbook Premium subscription though your Google Play Store Account:
See Google Play's accepted payment options.
If you do not wish to continue with your Playbook Premium subscription at the end of your minimum pay period, you are welcome to cancel your subscription. For more information about cancelling your subscription, please see our guide on how to cancel.
Your payment will be charged through Google Play Store at confirmation of purchase. The subscription automatically renews unless auto-renew is turned off at least 24-hours before the end of the current period.
We are unable to provide a refund for accounts that have been cancelled during their term, however we encourage you to contact us at firstname.lastname@example.org to discuss any matters or issues that you’re experiencing.