Yeah, but what in hell do I put there?

I read somewhere that for a person the importer number is the SSN + 00, but I'm not sure I can trust that info.
Yeah, you have to type your SSN (I understand your concern with this info), Tax ID or EIN (I use this) or Importer No. (you can get this from the CBP)
It's asking me for both importer number and SSN. SSN is not enough. I think I've found an answer I can trust:
"If you do not have an IRS business tax number, you may use your social security number in any paperwork requiring an importer number." From here:
https://help.cbp.gov/app/answers/detail/a_id/198/~/importer-numbersSo I'll fill in both SSN and Importer number with my SSN.