I’ve tested the free trial and I’m ready to buy Smart Logbook. 
To purchase Smart Logbook, open the Purchase screen in the app:
iOS:
More > Purchase
Android:
Side menu (≡) > Purchase info
Tap “Unlimited Flights” and complete the payment flow, which is handled by the App Store (on iOS) or Google Play Store (on Android).
After you purchase, you’ll be able to log as many flights as you like. The purchase can also be transferred if you want to use Smart Logbook on a different device.
For more info about purchasing Smart Logbook, check out these FAQs.