Google In-App Purchases Integration & Backend Setup

I need my Android app, built with Flutter, to have Google In-App Purchasing integrated for one-time purchases. The design for the app is already complete, so your focus will be on implementing the purchasing system and creating the necessary backend services for purchase validation.

Ideal Skills and Experience:
- Proficiency in Flutter and Android app development
- Experience with Google In-App Purchasing integration
- Backend development skills, preferably with experience in setting up purchase validation systems
- Understanding of one-time purchase systems within apps
- Good problem-solving skills for potential technical challenges