I know that not everyone in America is fortunate enough to be a native English speaker, but if you can't speak English then you shouldn't work at the Post Office.
I myself am not very gifted in the language department so I completely sympathize with people who have a tough time with English, but it doesn't seem unreasonable to require customer service people to know how to communicate with their customers. I've been to the same Post Office Jessica is referring to, and I think I've had the same cashier, but back when she was in training. The employee training her had to demonstrate to her how to physically take my driver's license from my hand and then how to return it to me because the woman seemed utterly incapable of performing even his simple task. At the time I was pretty confident she was mentally retarded. Which is fine, but again, she's probably in the wrong career path.