Stop using Zip Codes for geospatial analysis: https://carto.com/blog/zip-codes-spatial-analysis/, via https://www.metafilter.com/182600/humans-dont-behave-based-on-administrative-units
The problem is that they are designed to be convenient for the post office (geographically compact) rather than convenient for demographers (they sometimes combine areas that have quite different groups of people in them). The linked article is by a group selling software for one alternative subdivision that works better, but recommends several others as well.