When you initialize the Android Payment Links SDK, you need to provide a customerId. This allows the SDK to handle cases where the player closes the browser during checkout and later returns to the game. In such cases, the SDK begins validating the order using the customerId from initialization and the purchaseId from the checkout session. It then contacts the Appcharge server to check the order status and responds to the app with the next step. Additionally, the customerId ensures correct validation even if multiple players share the same device. For example, if a different player logs into the same game on the same phone, the SDK uses the customerId to identify the correct player and validate any associated orders with the Appcharge server. Use the following function to initialize the SDK: 
CheckoutController.Instance.Init("CUSTOMER_ID", callback)
ArgumentTypeDescription
Customer IDstringCustomer ID.
CallbackICheckoutPurchaseInterface callback.
If you didn’t configure the Checkout Public Key and Environment in the AppchargeConfig file, you need to add it here manually in the SDK initialization: 
CheckoutController.Instance.Init("CHECKOUT_PUBLIC_KEY", "ENVIRONMENT", "CUSTOMER_ID", callback)
Note: The second method is best when trying to change environment or customerId in real-time.
ArgumentTypeDescription
Checkout Public KeystringThe checkout public key, located in the Publisher Dashboard. In the sidebar menu, click Settings, then select the Integration tab. Copy the Checkout Public Key value.
EnvironmentstringThe checkout environment.
One of:
- sandbox: For testing.
- production: For live operations.
Customer IDstringCustomer ID.
CallbackICheckoutPurchaseInterface callback.