Hi!
RoboVM has now a new RoboPod called Robo.Billing which allows you to implement cross-platform in app purchases in your app:
There is no documentation yet, but you can take a look at the sample app:
Take a look at the the core project of the sample. You can do all in app purchasing in the core project, you do not need to change any code in your iOS project.
If you also want to let Robo.Billing handle Android IAPs you will have to implement some code in onActivityResult or your launch activity (just check the android project of the sample).
Also don't forget to add the necessary dependencies to your build.gradle:
core: compile "org.robovm:robopods-billing-core:1.13.1-SNAPSHOT"
ios: compile "org.robovm:robopods-billing-ios:1.13.1-SNAPSHOT"
android: compile "org.robovm:robopods-billing-android:1.13.1-SNAPSHOT"