The AirBattery does have an in-app purchase available if you want to get the pro version of the app. While the only unlockable feature at this time is the ability to see the battery life as an ongoing notification, the main reason why you want to pay the $0.99 is to remove the ads.
If you would like to pay for the pro version, head back into the app’s settings menu and select the Donate to remove ads, and gets the pro option. You’ll be taken to the Play Store to make the transaction and then be brought back to the app.
Now, when the buds overlay appears, you will no longer see the annoying banner ad. Additionally, if you head back to the settings menu again, you will be able to toggle Show notification on or off. When on, whenever your buds are paired, a notification will show the battery status. Additionally, you can customize how often that notification gets updated.