Georgia has 157 counties, more than any state except Texas, and only 952 postal codes to spread across them. That works out at 6.1 codes per county, the second-lowest ratio in the country after South Dakota, and it produces a specific shape of problem: almost every Georgia county is a handful of codes, so a county-keyed table is long while each of its rows is thin.
The consequence for fixtures is that sampling by city is nearly useless here. Atlanta, Savannah and Augusta between them touch a small fraction of 157 counties, and any logic keyed on county — tax jurisdiction, service coverage, reporting rollups — behaves differently in the 150 counties nobody sampled. Sampling by prefix is better, and Georgia has 22 of them.
Georgia also owns one of the country's odder postal facts, and it belongs on a fixture list. Atlanta's codes run in the 303xx block, but the metro's USPS accounting codes reach into 311xx and as far as 39901 — a single code attributed to Atlanta, sitting in a range that otherwise belongs to south-western Georgia. A prefix test written as "303 means Atlanta" misses real Atlanta mail, and a range check that assigns 399 elsewhere in the state misfiles it.
The prefixes are not contiguous, running 300 to 399 with substantial gaps, so Georgia needs a prefix set rather than a range. The largest by code count is 310 around Warner Robins rather than 300 around the Atlanta suburbs — territory over population again, and a reason not to guess which block is biggest.
And Georgia is the one US state whose name is also a country's. ISO 3166-1 assigns GE to Georgia the sovereign state in the Caucasus; ISO 3166-2 assigns US-GA to Georgia the state. Forms that merge country and region into a single searchable list, or that validate a two-letter code without knowing which standard it came from, will accept a Tbilisi postal code for an Atlanta address and never complain. It is the same lesson as Washington's: a place name is not an identifier, and the code is.