In Unity 2018.3, if you open Window > Package Manager, you can choose to install/remove "In App Purchasing".
This seems to be completely separate from the IAP version installed from the Services tab. If you only install IAP from the package manager, the Services tab doesn't detect that it's installed. If you install IAP from the Services tab and don't have it installed in the Package Manager, the Editor doesn't seem to care.
What does installing IAP from the Package Manager actually do?