Plann is available to purchase via the Apple iTunes and Google Play stores.
Technically, Plann is a product of each app store and subscriptions are managed via each platform.
If you have any questions about a receipt or need to change your payment details etc, please visit your profile with the respective provider.
Alternatively, here are more details on how subscriptions work.
Settings > iTunes & App Store > Choose your Apple ID > Subscriptions > Plann
To find out more on viewing, changing or cancelling Apple subscriptions, visit the Apple support page.
Google Play Store > Account > Subscriptions
To find out more on viewing, changing or cancelling Google Subscriptions, visit the Google support page.