How to auto-fill a county field based on zip code?

The file "2010 ZCTA to County Relationship File" on the following pages list ZipCode, State FIPS, and County Fips inthe first three columns:

https://www.census.gov/geo/maps-data/data/zcta_rel_download.html

A little script could reduce teh 44,000+ records to a QuickBse formula

State and County FIPS codes can be found here:

https://www2.census.gov/geo/docs/reference/codes/files/national_county.txt

But if you want accuracy be aware that zip code are just labels for postal delivery routes. Here in Michigan we have some places in western Upper Peninsula that have a Wisconsin zip code simple because that is the closest post office.