Son of a post office worker. John Edwards never tires of saying that he is the son of a mill worker. Why not vary the theme occasionally and say that he's the son of a post office worker? Does the job your father did make you who you are, while the job your mother did is either a side note or the bare fact that you had a working mother? And of course, "working mother" still reads as deprivation, quite the opposite of "working father."
Edwards' official website, by the way, highlights his mother in terms of pie. No, not apple pie, as in the politician's favorite "mom-and-apple-pie." It's peanut butter pie. In fact, they'll send you one for $50.