Sometimes, pissing may not work. Most likely it's biting with the Double Kick mod. Check if you have Super Double Kick installed, open "ShellModifier.ini" in the "\ POSTAL2Complete \ System" folder. Then add the semicolon at the beginning to make it look like this:
;--Double super kick
;Plugins=class'DoubleFoot.sM_DoubleKickPlugin'

You can also simply remove these two lines of text from the file.