I have 9/10 postcards in Traverse Town in the original Kingdom Hearts. I am pretty sure the last one I was missing is the one postcard you get when you go to the roof of the Gizmo Shop, run across the house roofs, and enter a door to the third district to target something.
However, when I go to that door atop the roofs in the second district, I can't go through it. It displays the text "Third District" in the bottom of the screen like it does whenever you are about to go to another screen, but I cannot walk through the door. It just acts like it is a regular wall.
I don't know if this affected the problem at all, but a long time ago I had found that door when exploring Traverse Town and I remember being able to go through it, but at the time I didn't know there was a postcard so I don't think I targeted anything. Does going through the door once lock it or something?