I'd like to avoid unwanted purchases.

You can prevent unintentional or unauthorized purchases by blocking in-app purchases or setting passwords.

<iOS>
Use Restrictions to protect your iTunes Store account:
1. Go to Settings > General > Restrictions.
2. Tap Enable Restrictions.
3. To prevent in-app purchases, turn off In-App Purchases

How to Turn on Restrictions on Apple device



<Android>
Set up authentication within the Google Play Store app.
1. Touch the Menu icon > Settings.
2. Touch Require authentication for purchases.
3. Follow the on-screen instructions and choose a setting.

How to prevent unwanted purchases on Android devices