From the Apple dev docs:
> Apps utilizing a system other than the In-App Purchase API (IAP) to purchase content, functionality, or services in an App will be rejected.
And:
> Apps used for financial trading, investing, or money management should come from the financial institution performing such services or must use a public API offered by the institution in compliance with its Terms & Conditions.
And:
> Apps that provide services in highly-regulated fields (such as banking and financial services, healthcare, and air travel) or that require sensitive user information should be submitted by a legal entity that provides the services, and not by an individual developer.
> 3.1.5(a) Goods and Services Outside of the App: If your app enables people to purchase goods or services that will be consumed outside of the app, you must use purchase methods other than in-app purchase to collect those payments, such as Apple Pay or traditional credit card entry.