The current "Premium Service" is a subscription.
You can choose between a monthly plan and a yearly plan, and it renews automatically until you cancel.
If you purchased the one-time "Remove ads" option in an earlier version, you can continue to use the app without ads at no extra charge. Your ad-free access will not be lost even if you do not subscribe.
Also, with the same Google Account, a single subscription covers Premium features on multiple Android devices.