Cross-Platform Startup App Development

I’m building the first version of a brand-new mobile app for my startup and need someone who can take the idea from concept to a polished release on both iOS and Android. The scope is a full cross-platform build that handles secure user authentication and a seamless payment gateway from day one... (Budget: $1500 - $3000 USD, Jobs: Android, API Integration, Documentation, Flutter, iOS Development, iPhone, Mobile App Development, Payment Gateway Integration, PhoneGap, Software Architecture)