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In the typical primary database in layers window, under more-> US Govt, there is a Postal Code Boundaries layer. I have been trying to find a way to save this, and ultimately add it to a folder of my own.
Another caveat: I want to save it in KML form, so that I may open it up in notepad and extract the coordinates making the boundaries to save in a database.
The Postal Code dataset in Google Earth Pro appears to be somewhat out of date in certain areas, and there isn't a way to extract the data from Earth's built-in layers, so I would suggest going to the source for this data.
Searching data.gov for "postal code" brings up many datasets for different geographies, and time periods. I suggest looking through there to find what you need.
Note that, if my admittedly limited understanding is correct, post codes or zip codes are not originally polygon/boundary data. Apparently the Postal Service defines zip codes by the street and range of address numbers (which makes sense for how they use them to deliver mail), so they're really a set of linear features. Anywhere that you see post code boundaries or polygons, most likely someone has tried to estimate where the divisions lie between the streets. This is usually along property lines, but often is based on various assumptions.
Approximate US postal code polygons are available from census.gov. The data you want is called ZCTA (Zip Code Tabulation Areas). This is the closest approximation to real postal code areas you can get without spending huge amounts of money. It's good enough for most purposes. The most recent are probably from 2010, since that's when the most recent census happened. The 2020 census will start soon, so there will be more current data available in the near future.
See these questions for more information about zip code polygons:
