Hi, just to clarify: In-App purchases have to call a specific API from a store that you want to use. The most popular options for that on PC are Steam, Microsoft Store and Epic Launcher. So you would have to find a library or Unreal plugin that connects to the ones that you want to use. I did a quick search on the market place but I only found this plugin for Steam, and none for the others:
Note that the description does not say if it supports in-app transactions, so this needs to be checked.
I would expect there is a solution for Epic Launcher as well since Epic probably wants to make money from this 
But I found nothing on the market place. Maybe it’s included in the engine by default?