You can avoid accidental purchases by changing your device settings.

For Android devices:

1. Open the Play Store app.
2. Tap the Menu icon and select Settings.
3. Tap Require authentication for purchases.
4. Select one of the authentication options.
- For all purchases through Google Play on this device. Authentication will be required for all Google Play purchases (including in-app purchases).
- Every 30 minutes. The system will request the password for Google Play purchases every 30 minutes. Just note that once you enter the password all purchases made over the following 30 minutes will not need a password.
5. Follow the instructions on the screen.

For Kindle devices:
1. Open the Amazon Appstore application.
2. Go to Menu > Settings.
3. Untick the In-app Purchasing option.