I spent hours trying to get this working, and it turns out it was fairly simple in the end.
Firstly, you want to install AVPC under the ShareThePain directory, NOT the main directory. That no longer works. So when installing, install to:
Next, to get it to show up on the new Launcher, go to your main directory:
Find the file named Launcher.ini and add this to the bottom of it:
Name=A Very Postal Christmas
That's it. One other thing to do, if you're getting crashes, is turn off Allow Projectors in the Performance options. For some reason AVPC always crashes after 30 seconds or so ingame when that's checked. For me anyway.
This could work for other mods. Haven't tried any.