For one, how would a hearing dog assist you in the PO???
Dont get me wrong I think hearing dogs for the deaf are wonderful - IF USED FOR THE RIGHT REASONS.
If I lived by myself yes I would absolutely consider having a service dog to alert me to the door, alarms, and other things in the home. But out and about around town?? You just have to be more aware of your surroundings. a hearing dog cannot be trained to alert you to an ambulance running hot on code 3 behind you and approaching at 60 mph in a 45 mph zone. For one sirens tend to scare the crap out of most dogs, secondly repeated exposure to sirens for training purposes kinda makes the dog deaf...... so then you'll need a service dog for your service dog for you.
Back to seriousness - I once again agree with service dogs but you have to use them for the right reasons, not just a free house trained pet that can do a few tricks that you can also take with you to wal-mart. (Sadly there are people out there that do just that).
The best place for a hearing dog to truly function is in the home (including the yard, alerting when someone drives in the driveway) and some cases in an office setting.
You dont need a hearing dog to get pick up your mail - the blind do - especially those that live in bigger cities with high traffic and those that are deafblind. A person with severe epilepsy needs a service dog to alert them to a possible seizure, a seizure can happen at the PO. A person with certain forms of dementia may need one in case they become disoriented and the dog needs to be commanded to lead the person outdoors so they can regain their bearings. (IE a person that happens to have panic attacks in places with lots of people), a person with a muscular problem that can walk but also falls frequently needs a 'paw' in getting back on their feet, a wheelchair bound person that has limited mobility of the upperbody to reach down to pick up things and to manuever. These would be right reasons to have a service dog in the PO.
I dont need to talk to someone to obtain my mail at my PO box - 99% of the time I buy my stamps from the machine, and if I do have to send a package over the counter I can write out what I want and me and the clerk can write back and forth. USPS is very accomodating to the deaf IMO. They have been known to hire deaf people to work for them, I dont see this as a discrimination but perhaps there is a blind person that has a guide dog that uses your PO and another person saw them with their guide dog and thought it would be OK to bring Fido into the PO then the next person sees Fido then they have to bring their Fluffy and it goes on and before you know it everyone is bringing their dog to the PO and they are leaving a mess behind and it gets out of hand. So the postmaster has to post a sign but like aforementioned, probably did not consider the other service dogs out there when he wrote the sign, and was not intentionally discriminating.
Sorry to be the devil's advocate tonight but thats my humble opinion on the whole.