"Premium Service" is currently offered as a subscription. You can choose between a monthly plan and an annual plan, and it renews automatically until you cancel.
If you made a one-time "Remove Ads" purchase in a previous version, you can continue to use the app ad-free at no additional cost. Your ad-removal purchase will never expire, even without subscribing.
Also, if you use the same Google Account, one subscription lets you use the Premium features on multiple Android devices.