Due to the way the App Store works you cannot be charged for anything without your explicit permission.

The subscriber is charged automatically at the end of the trial period. According to the App Store policy, subscriptions automatically renew unless canceled at least 24 hours before the end of the current subscription.

We try to make this as clear as possible by mentioning it both in the description and inside the app itself when the premium offering is shown.
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Thank you for your understanding.