I upgraded one of the free apps in the past, but now it's asking me to pay again

If you already purchased an in-app upgrade, you may need to restore it if you purchase a new device; or you install the app on a different device that uses the same app store account; or you deleted and reinstalled the app; or your device unexpectedly loses its association with the upgrade. You will not be charged again when you restore the upgrade.

To restore the upgrade:
  1. Navigate to the Settings tab in the app.
  2. Click on Restore In-App Purchase at the bottom.
The App Store has very effective safeguards to prevent duplicate purchases. Even if you try to buy an in-app upgrade a second time, the final message will notify you that you already own the item, which will automatically be restored right then. So there's no risk of being charged twice for the same item, as long as you're using the same account that you used to purchase it originally.

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