Notifier is created to let you know when a customer sends or updates a ticket. The app sorts notifications by priority or any other filter. For more information visit: https://ontecapps.com/en/apps/notifier.
Notifier costs $0.75 per month. A trial is free, but when you install the app, you will still be asked to enter your card details - this is a Marketplace requirement. However, no charges occur until the 14-days trial period ends.
You might find this article helpful when the app is installed: How to enable notifications on new tickets
If you want to avoid charges, you need to uninstall the application before the end of the trial period as described here: How to uninstall an application and cancel its subscription.