TempFly
TempFly is a highly configurable spigot/bukkit plugin for minecraft that introduces many flight related features to the game. Players can be given flight time that is used when they fly and can be payed to other players like a currency.
Developers:
TempFly can be hooked into using the TempFlyAPI. this can be aquired using TempFly.getAPI(). There is lots of documentation in the source code that explains what to do but if you still have problems i made a small tutorial