With Overpass you have the option of paying for unlimited data for a set of applications over a particular time period (e.g. 1 day, 1 week, 1 month, etc). While some offers are promotional and allow you to make a one time purchase, most offers are subscriptions and so you'll be charged on a recurring basis for the amount and frequency specified in the subscription.
If at any time decide you no longer want the Overpass subscription, you can cancel them via Apple and Google's services.
See https://support.google.com/googleplay/answer/7018481 for how to cancel an Android subscription.
See https://support.apple.com/en-us/HT202039 for how to cancel an Apple subscription.