Flutter App Dev - Extensive Customization

I'm looking for an experienced Flutter developer who can craft an app with a professional and formal user interface, and help with extensive customization. The key features are as follows:

- In-app purchases: Integrating this feature is important to facilitate transactions within the app.
- High-level Customization: There will be a need for an assortment of unique custom features and design elements for this project.

Ideal skills include:

- Proficiency in Flutter
- Familiarity with professional and formal UI design
- Excellent understanding of advanced customization techniques
- Experience integrating in-app purchase features

Please share your portfolio of previous Flutter-based projects.