Did they give up the rights to the Postal series or lose them?
As far as I know they own the rights still but licensed them to a third party for the creation of the Postal III game which was widely considered very disappointing at large. They even stopped selling that game in their own online store and had harsh words to say about the game as well which are still on their website.
What's extra awesome and funny though, is that they greatly enhanced these games in the last year or so with all kinds of new content, in particular Postal 2, including adding content that you can find a copy of Postal III in a junkyard and if you pee on it you get an achievement or some other benefit. :)
They're also working on a new expansion for Postal 2 that was supposed to be out already I think but not sure if it made it out yet or not. They said that we'd be getting that on GOG.com as well last year.
So I imagine they could expand the series on their own if they felt up to it, a Postal 4 or something or other perhaps. With a small team taking so long to do the Postal 2 expansion though I imagine it might take years to see any new titles from them, but it'd be awesome if they went for it and put out something successful. Maybe even a kickstarter or something. :)