First-of-all, let us see what a Freemium app is. A Freemium model is a mix of an application's free and premium versions.
Freemium apps are free to download. Once you download the app, you get a set of features that are free to use.
The advanced features are usually locked and you'll have to purchase the Premium membership to access them.
Similarly, GoJournal is free for download in the AppStore. You can enjoy the handwriting features, unlimited entries, and various other functions within the free version.
But to get a complete app experience along with unlimited journals, custom template creation, and more, you will have to make a one-time in-app purchase of the application. And yes, after the one-time purchase, no additional in-app purchases are needed in the future again. All the current & future Premium features that will be added to the application will be unlocked for you automatically.