Grid Note's Premium is enabled via the auto-renewing subscription on iOS.
Making a purchase
Payment will be charged to your iTunes account at confirmation of purchase. All purchases are managed through your Account section in the Mac or iOS App Store. Once you purchase a Premium subscription, all premium features will be available to you.
Your subscription will automatically renew unless auto-renew is turned off at least 24-hours before the end of the current subscription period.
Your account will be charged for renewal within 24-hours prior to the end of the current subscription period. Automatic renewals will cost the same price you were originally charged for the subscription.
Manage the subscription
You can manage your subscriptions and turn off auto-renewal by going to your Account Settings on the App Store after purchase. Please see https://support.apple.com/en-us/HT202039 for more details.