Cross-Platform Online Shopping App Development

I'm looking to develop an online shopping application that works on both mobile (Android first) and web platforms.

Essential features include:
- User authentication
- Payment integration

Ideal skills and experience:
- Proven experience in developing cross-platform applications
- Strong background in mobile app development, particularly on Android
- Expertise in implementing secure user authentication and payment systems
- Knowledge of UI/UX design principles for a seamless shopping experience

Please provide examples of previous work and estimated timelines.