iOS and Android E-commerce App Development

I'm looking to develop an iOS and Android e-commerce app. The app should include payment gateway integration and user authentication.

Key Requirements:
- Payment gateway integration
- User authentication


Ideal Skills and Experience:
- Proven experience in developing iOS e-commerce apps
- Strong knowledge of payment gateway APIs
- Expertise in implementing secure authentication methods
- Familiarity with Apple's App Store guidelines

Please provide samples of similar work done and an estimated timeline for completion.