Seattle-based Amazon said it has added a new plugin which makes it easier for Kindle Fire developers using the Unity game engine to add in-app purchases to their games. Amazon.com said it has added a new plugin for developers using Unity to add in-app purchasing and GameCircle features to their apps running on Kindle Fire and Kindle Fire HD. Unity is a 3D game development tool, aimed at video game developers. Amazon has been offering up in-app purchases since earlier this year. The firm's GameCircle APIs help develops create leaderboards, achievements, and other social features within their games.