Promo codes, Google Play's billing system

Google Play FREE REDEEM CODES - AWBI

Android Developers Blog: Create promo codes for your apps and in-app products in the Google Play Developer Console, promo codes

Why can't I create a promo code? - Google Play Developer Community

How to use Promo Code on Google Play Store and App Store - DexAlert

How to Create Google Play Promo Codes for Android Apps

Google Play

Google Play's billing system: Update

Own promo codes (vouchers) for free and/or paid access - Google Play Developer Community

Promo codes, Google Play's billing system