in-app purchase
https://developer.android.com/google/play/billing/billing_overview
https://developer.apple.com/in-app-purchase/
https://viblo.asia/p/in-app-billing-tren-mobile-va-cac-quan-diem-test-63vKj60xK2R
https://viblo.asia/p/in-app-purchase-ios-ZabG9zzwvzY6#_luong-thanh-toan-cua-apple-2
https://hackernoon.com/swift-how-to-add-in-app-purchases-in-your-ios-app-c1dc2fc82319
https://developer.apple.com/in-app-purchase/
https://www.raywenderlich.com/194869/in-app-purchase-tutorial-getting-started
IAB Android
https://www.youtube.com/watch?v=nn2IbTSYNzA
https://developer.android.com/google/play/billing/billing_overview
https://viblo.asia/p/in-app-billing-trong-android-lam-giau-khong-kho-part-1-implementation-maGK7zqO5j2
https://viblo.asia/p/in-app-billing-trong-android-lam-giau-khong-kho-part-2-work-flow-6J3ZgEGL5mB
http://blog.andolasoft.com/2014/05/how-to-monetize-your-android-application-through-in-app-billing.html
IAP Unity
https://github.com/onepf/OpenIAB-Unity-Plugin
https://www.youtube.com/watch?v=xy59by6FoMY