Here you go |\/|ILF.
My stupid map.Pleas fix and send it to me.
Hehe. Don't get too stressed out.
I would be glad to fix the map for you, (if I even could) but unfortunately at this moment in time I am keeping my PostalSTP installation 'clean'.
That is to say, I have removed any M@d/TN/AW mods and textures from my Postal install. Because I am fiddling around with some multiplayer maps. And it is all TOO EASY to accidentally use a non Postal stock texture in a map. This causes problems for sharing later of course. If others do not have AW for example, installed. (Makes your map less shareable if you don't place them non stock textures in 'MyLevel').
I know you said that this map is to be single player, so that is fine as long as the people that want to use it, know and have AW7 installed. (If you are planning on sharing/uploading it anywhere, it might be a good idea to indicate somewhere, that AW7 is required. Possibly in the name as well as in a ReadMe file).
So my point is, I can't open your map at the moment, because you have used some AW stuff that I don't have installed at present. (Got an error about an AW7 door mover or something).
But, if your only problem at the moment is the mirror thing. Just replace the 'FinalBlend mirrorcrud' with something like Texture grey-128
(from the same package) or something. Then right-click, choose Surface Properties, tick mirror (it may even be ticked already).