Premium and Ultimate are available as auto-renewable subscriptions (monthly or yearly) purchased through the App Store or Google Play.
Your payment will be charged to your Apple/Google account and will automatically renew unless canceled at least 24 hours before the end of the current period.
You can manage your subscriptions and auto-renewal may be turned off in your Apple/Google Account Settings after purchase, however, you will not be able to cancel the current subscription cycle once it is activated (e.g. the current month or year).
Manage your Apple subscriptions: https://support.apple.com/en-us/HT202039
Manage your Google subscriptions: https://support.google.com/googleplay/answer/7018481?co=GENIE.Platform%3DAndroid&hl=en