You can use it in all your projects with the same Unity ID exporting the final product. But if you need to use another Unity ID, you need to purchase a new license for it.
All source code of the Unity side C# and native .m/.java/.cs files are included in the package. You can modify these code to fit your project better, but you should observe the EULA of Unity Asset Store you agreed when you register your Unity Asset Store account. That means you have no rights to reproduce, duplicate, copy, sell, trade or resell this package.
If you need to redistribute your package containing UniWebView to other developers (that means you are developing a SDK or something else which is not orientate to final customers), you need to purchase a Limited Redistribution License instead.