Prices are one-time payment. No monthly fee is required.
Open the app and sign-in with your Google account, then move to the license management page.
In the license management page, you can find purchase buttons for each license. You can purchase licenses via Google Play Store.
Licenses are tied to your Google account, so you can activate any of your Android devices by sign-in with the same Google account.
When the app is no longer used on a device, you can deactivate the device and the license will be returned to the original Google account. Thus you can reuse your licenses.