Great question — answering as the UphillTech team:
EDIT 2: I found yet another source that proves my answer right. IATA Travel Centre: https://www.iatatravelcentre.com/MX-Mexico-passport-visa-health-travel-document-requirements.htm I filled the form with a travel from Mexico to Mexico, 10 days (to avoid any VISA issues), adult traveling with a US passport card. The results says it pretty clear: Summary: No , the documentation you hold is NOT sufficient for the journey you have specified. Passport: Passport required. You can fill it out yourself to see the results. PS. I got redirected to IATA's online tool by Aeromexico, the flag carrier airline of Mexico. EDIT: The answer stays the same. The passport card is not valid for air travel except...
Full answer by NoName: https://travel.stackexchange.com/a/166386 (CC BY-SA 4.0).
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