If you need to support in app payments in your IOS app you may want to take a look at this component for Delphi Firemonkey. It is a commercial component but it will probably save you time in the long run because you won’t have to try to access StoreKit yourself. It supports the consumable and non-consumable purchase types from IOS but does not support IAP Subscriptions which is a drawback. You can download and test it out with a trial version which is a big plus. In App Payments allow you to have an additional revenue stream in your app instead of just being able to charge for it in the App Store. You app could be free and then you offer premium options within the app using the in app payments.
Check out the component over on their site and download the free trial.