ZCTA by County (geo within geo)

I am trying to measure population by ZCTA within a county, but only in that county, meaning that inter-county ZCTA population is not included.  I know the old AFF used to give us geo within geo option, but am not sure it would have helped with my inquiry.  The current data.census.gov will not parse out ZCTA within respective counties to best of my use, which, unfortunately, I'm never sure I'm utilizing as census envisioned.

Is there an non-GIS (dissolving shapefiles) way to estimate my query?  I hope this is clear.

Thanks

Eric