This is how the free trial works in the Google Play Store:
- If you accept a trial offer, you'll get an email when your trial is about to end.
- At the end of your trial period, the first billing period will begin and you will automatically get charged the amount in your subscription terms.
- To avoid charges, cancel the subscription before the trial period expires.
- You may find a payment authorization on your payment method for up to 24 hours before the next billing period starts. However, you’re only charged on the billing period renewal date. To remove the payment authorization, cancel the subscription before your next billing cycle begins.
For more information on free trials and subscriptions, please visit this webpage: https://support.google.com/googleplay/answer/2476088