It takes months to implement in-app purchases for Unity even with a native solution.
That requires you to write a server and ETL for subscription and purchases data.
Both SDKs written in pure Swift/Kotlin and support iOS 9+, Android 4.1+ which fit for 99.9% users all wrapped into C# lib.
5 SDK methods are all you need to deploy IAPs and get subscription data from Apple and Google without coding.
It takes only a couple of hours for a junior developer to implement IAPs with Adapty Flutter Plugin
Make a purchase with receipt verification with a single method. Apple Promotion Offer supported right out of the box.
Restore purchases when a user re-install the app.
Link transaction to a user ID in your system.
Set any number of custom user attributes and set a separate IDs for each analytics/attribution services.
Link a user with transactions to an attribution such as ad source, campaign, ad, creative. AppsFlyer, Adjust, Branch, and Apple Seach Ads supported.