Joe. I don't have a detailed web page, but there's a related application at http://mcdc.missouri.edu/applications/zipcodes/.
The data sources are the Free Zip Code Database at http://federalgovernmentzipcodes.us/; the USCB's Gazetteer files at https://www.census.gov/geographies/reference-files/time-series/geo/gazetteer-files.html; and MCDC's own ACS profiles, based on the 2014-2018 ACS 5-year release.
Our app keeps the ZIP code type, state abbreviation, and location text from the Free Zip Code Database. I also filter out decommissioned ZIPs. PRIMARY ZIP types become the "preferred" name, and ACCEPTABLE ZIPs are combined to become the "alternate" names.
I use the USCB gazetteer file to add land area and a more precise lat/lon.
I join the most recent ACS 5-year ZCTA data to add the population, housing units, and median family income.
The MFI percentile rank is a little more complicated, but I'm happy to share my algorithm with you if you like.
As for why some PO Box ZIP types include MFIs, that's a very good question. I see 2,330 such in my database, out of 41,276 in all. I'm not sure what that's about. I'll have to get back to you on that.