Coded

When the user wants to buy a product call the InitiatePurchase method of the IStoreController, identifying the product the user wants to buy.

// Example method called when the user presses a 'buy' button
// to start the purchase process.
public void OnPurchaseClicked(string productId) {
    controller.InitiatePurchase(productId);
}

Your application will be notified asynchronously of the result, either with an invocation of ProcessPurchase for successful purchases or OnPurchaseFailed for failures.