You're misunderstanding what I've said.
a) edit the current Georgia state in your zones list. Change it to "Georgia: Marietta County"
b) then add MORE states for Georgia: one for each county: "Georgia: Appling County", "Georgia: Bacon County", "Georgia: Fulton County", etc ... all 321 of them.
Then configure taxes for each "state" separately.
Done: customers can now easily choose their county, and tax will be applied correctly
Bookmarks