Is In-app upgrade (Premium Version) a one-time charge?

Yes. Our In-app upgrade to Premium Versions in all of our Apps is ONE-TIME only charge. It's NOT a subscription. You will only be charged the first time you purchase the upgrade and enjoy it for ever on all your devices. If you re-install the App or install it on another device, you can restore your upgrade on the Upgrade Screen. (Tap "Restore" button). You will not be charged again when you restore a previous purchase.