Just noticed this in data.census.gov.
The Census Bureau does not release ACS data at the ZIP Code level, but instead for ZIP Code Tabulation Areas (ZCTAs), which are generalized representations of U.S. Postal Service ZIP Code service areas…
It may be a little bit out of date, but I wrote this blog post about the difference a few years ago: www.policymap.com/.../what-are-zip-code-tabulation-areas