I had no problems a few years ago (2009), but I figured that if I ran into trouble at one SSA office, I would try another, perhaps after trying again on another day.
Nowadays, I would ask point blank to speak to the clerk's supervisor.
With that said, make sure that your letter looks exactly like their form letter (it would be nice to have that typo fixed; the word should be "foregoing," not "forgoing"). If any key information is missing, such as the doctor's medical license number, then you can be refused. If your letter is appropriate, then kick some ass.