How do I stop the free trial so I’m not charged?

Cancel it in the store before the three days end.

• On iPhone, at least 24 hours before the end: Settings, your name, Subscriptions, Berceuse, Cancel Subscription.
• On Android, before the end: Play Store, your profile, Payments and subscriptions, Subscriptions, Berceuse, Cancel subscription.

You keep Premium until the end of the trial, and you pay nothing.