I had the same issue here is how I fixed it go to your directory where postal 2 is installed then go to the "System" directory then find the "Postal2.ini" file and open it in notepad in the section marked "[Engine.Engine]" you'll see three lines right underneath it the first one will say "RenderDevice=D3DDrv.D3DRenderDevice" put a semi-colon in front of that ";" then remove the semi-colon on the one that says "RenderDevice=OpenGLDrv.OpenGLRenderDevice". It should look like this:
Save the file and run the game it should work.
Or, (This info is from a User named Gothik13) If you still want to run in D3D there is another fix. In the Postal2.ini file go to the [D3DDrv.D3DRenderDevice] section and under that change ReduceMouseLag=True to ReduceMouseLag=False Save and start the game. It will now run with D3D. However, I've found the game will still crash if you try to take screenshots.