You can purchase a Playbook Premium subscription though your iTunes Account:
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 to iTunes Account 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.
Your subscription can be managed/canceled by going to user’s Account Settings in iTunes after purchase. Any unused portion of free trial period will be forfeited when the user purchases a subscription to that publication.
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.