HandleError returns error message by status code.

New creates a client object.

NewWithConfig creates a client with configuration.

Subscription was canceled by Apple customer support.

Customer changed the plan that takes affect at the next subscription renewal.

Initial purchase of the subscription.

Customer renewed a subscription interactively after it lapsed.

Automatic renewal was successful for an expired subscription.

ProductionURL is the endpoint for production environment.

SandboxURL is the endpoint for sandbox environment.

Client implements IAPClient.

Config is a configuration to initialize client.

IAPClient is an interface to call validation API in App Store.

https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/StoreKitGuide/Chapters/Subscriptions.html#//apple_ref/doc/uid/TP40008267-CH7-SW16.