First, the ADA does not apply to post offices, but the Rehab Act does. Second, in response to the poster that mentioned "certified service dogs", in the US, service dogs are not certified in the United States. No certification, identification, or registration is required.

I just had a problem at my local post office. I filed a complaint and the supervisor defended the employee and said, "Well, you said he isn't a guide dog". He isn't, he's a medical alert dog. He said, "Oh, was he leashed?" I replied that he was both leashed and harnessed and he replied, "As long as he was harnessed". WRONG AGAIN BUDDY! In the US, service dogs often do, but are not required to be harnessed. Mine is because he does mild mobility work, but it's not required.

What is required:

1) The handler has a life limiting disability
2) The dog does something to mitigate this disability
3) The dog is trained to behave properly in public!