In-app purchases
There are different solutions for in-app payments for iOS and Android. Neither solution works on the other platform, so if you are releasing your app to both platforms, you will have to integrate code for two modules. For iOS you should use StoreKit and for Android, In-App Billing.
Tip
You can also use the free PayPal module for collecting payment. It's available for Android and iOS. However, Apple does not accept payment through any means other than in-app purchases for extensions and subscriptions for your app. You can only make payments for services such as picture postcards or goods via PayPal.
For this example we are going to add an 'exotic currency' purchasable item to the forex app.