Hi 
You can install an In App Purchasing package in the Package Manager and also update In-App Purchasing through the services tab. The update in the service tab seems to be the same as Windows > Unity IAP > IAP Updates.
What do I need to install?
Based on the version numbers they are different things but at face value they appear to be the same thing. Very confusing. Can someone please explain.
Thanks
If you want to use Unity IAP, you will need to install both.
Unity IAP has always been in two pieces, but one of them was built into the Unity Engine. Now that we are moving things out of the Engine and into Package Manager, that piece is less hidden.
The reason for the split is two fold: Integrating part of IAP (the Core) into the Engine makes integration easier; however, the IAP Plugin (that is from the Asset Store or Services window) contains all the wrappers for the app stores, which need to change more often than Engine. Now that Packages can be updated separately, they might be combined at some point in the future. (Though we would always want to maintain backwards compatibility.)