|Postal 3 - User-Made SDK|
I was able to come up with an unofficial SDK using a custom configuration in Source SDK 2009, and the decompiling of maps to add npcs and other entities.
I will later give details on how to do this unless the official SDK is released before I do.
Edit: 1:04AM - 7/7/12
I'm working on a tutorial page on my website on how to set this up, it should be up sometime next week.
Edit 1:52AM - 7/8/12
Nevermind, instead of a week here it is now: http://diodesoftware.webs.com/postalsdk.html
Edit 1:35AM - 7/31/12
Konstantin Efimov emailed me the original FGDs. Go here for the revised tutorial page and download them there.