I live in the 30345 zip code. I get mail addressed to "Atlanta GA 30345"; "Decatur GA 30345"; "Northlake GA 30345"; "GA Address 30345"; and probably some others I don’t remember. What does that make me “part of”? Nothing! If someone wanted to send me mail, and address it “Emory GA 30345”, it would get here.
Some day a marketing genius will start sending out junk mail addressed to “Target GA 30345”, or “Macy’s GA 30345”; or maybe “Coca Cola GA 30345”. It will get here. Please forward me the royalty checks on that idea.
The Post Office doesn’t define legal boundaries of anything; and in fact, doesn’t care what you put as the “city”. They read the zip code and the street address; it’s only if there’s no match that they have to look at the city name to try to figure it out. (And BTW, if the mail piece is bar coded across the bottom, they don’t even have to do that much.)
What about the other way around? Should a city (or proposed city) try to follow zip code boundaries in its city design? No. Zip code boundaries have the same significance as the delivery zones of Domino’s pizza chain. If you’re “here” on the map, you get your pizza from this store; if you’re a block away, it would come from another store. Zip codes are a delivery zone for Post Office “stores”; that’s all.
When we were talking to Emory University, the marketing question came up. They market themselves as being in “Atlanta GA”. (They’re not of course). We pointed out that if you look up their SEC reports, you’ll find that so does UPS (in Sandy Springs), Newell Rubbermaid (in Dunwoody), and Home Depot (in unincorporated Cobb County). The city name you use to identify your business is a marketing decision, not a postal decision.