Hmm, which leads to the question...Do ZIP codes belonging to dead post offices get recycled?
28335 is used by a former Dunn PO, now it is merged with 28334. Old Post office was 28335 and the new one on Clinton Ave was 28334. Nevertheless, the PO in Dunn uses 28334/5 to maintain this. 
Not quite what I'm talking about—it sounds like the 28335 is simply being served out of a new post office building that also serves another zip. I'm talking about a situation where Bertville, zip 73271, has its button copy factory close down and the town loses enough population for the PO to close. Then later, a new suburb elsewhere in the state called Erniedale merits a PO for the first time, and they assign 73271 to it.