Thanks for the tip, Jeff!
I ended up deleting UnityPurchasing & UDP folders in my main project. Then I created a new Unity project and imported the IAP package from Services window and moved the files to my current project manually (folders Plugins/UnityPurchasing and Plugins/UDP). This worked and my project is now without errors and IAPs/Ads seem to work fine in a build.
It seems the installation got stuck on whatever in my main project, and didn’t properly import the needed items & remove the installation files (see blank installation window problem), as those were still present after installation was “completed”. Using Perforce, which locks files not checked out, might have something to do with this but I was sure to check out everything related to the file locations used.
I was blatantly lying
in my earlier post, as I think I first installed the IAP package from the Asset Store and not the services window. Installing from Services in a new project & moving the files manually seemed to work, but installing the Asset Store version and doing the same didn’t. I had to resort to this because anything I did in the main project just caused more errors.
Regarding manual file deletion, I was following this topic: https://discussions.unity.com/t/687770
Is this info stated clearly somewhere? Let me get this straight…
If I install from Services, it includes everything I need?
But if I install from Asset Store, I also have to install a package from the Package Manager?
But installing from both Package Manager & Services causes overlap errors?
Honestly I think this process should seriously be simplified, as Unity IAP and Monetization packages have been a huge hassle every time we’ve had to Install or Update these packages. I’m not expecting an answer to these, just stating some inconsistencies with the package:
- Why do we have three different locations to download the IAP package from? Is this really necessary especially considering the overlap problems caused by accidentally installing the package twice from different locations?
- What is the difference between IAP from Asset Store, IAP from Services and IAP from Package Manager? This is not very clear, but I’m guessing they’re the same package?
- Honestly the difference between the IAP and Monetization packages isn’t very clear from reading the store pages. Why aren’t they bundled together, seeing as they both ultimately have something to do with monetization? It’s not clear whether or not both are required to create a functioning IAP/Ads combo for mobile.
- Most of the documentation found on Google seems to lead to legacy versions of IAP & Monetization. To be honest I didn’t even bother to check these while dealing with this new problem, as the last time I had problems with these packages the documentation just made everything more confusing.
I know making games requires some technical expertise, but this just seems way too convoluted for a rather simple procedure of installing a plugin. 
Thanks anyway, hope this might help some people dealing with the same issue. I’ll be sure to report back here if any more issues arise regarding the upgrade!