Hittade denna information om ESTA på
https://esta.cbp.dhs.gov/esta/WebHelp/ESTA_Screen-Level_Online_Help_1.htm
Detta gäller ansökan om ESTA
"When do I need to apply for a travel authorization?
Applications may be submitted at any time prior to travel to the United States. The Department of Homeland Security recommends that travel authorization applications be submitted at least 72 hours prior to travel. Unless revoked, travel authorizations are valid for two years from the date of authorization, or until your passport expires, whichever comes first."
72 timmar är tydligen ingen regel utan en rekomendation.
"ESTA will accept applications from last minute and emergency travelers – those Visa Waiver Program travelers that arrive at the airport without an approved ESTA."
Så i princip ska du tydligen kunna dyka upp på flygplatsen och fixa ESTA ansökan. Själv skulle jag nog undvika det av rädsla för att något ska strula.