Cross-Platform Lemonade Loyalty App

I run a small lemonade stall and I want to give my customers a fun, digital way to collect stamps and earn free drinks. I need a simple but polished loyalty reward app that works natively on both iOS and Android... (Budget: $30 - $250 AUD, Jobs: Android, Android App Development, App Design, Graphic Design, iOS Development, iPhone, Mobile App Development, UI / User Interface)