Once you have downloaded the free app, you can buy SmartFlybox from within the app.
To purchase a license for SmartFlybox, touch the menu button (top left button) and choose the "Purchase" section:
Touch the license item to go to the iTunes Store to purchase the license.
If you have already purchased a license, you will see this shown on the screen:
It is not possible for you to buy the license more than once, as long as you use your Apple ID to log onto the iTunes Store.