While one may book a Greyhound bus ticket on the same day for a trip within the United States, It seems that one cannot book a Greyhound bus from the United States to Mexico departing within the next 3 days (I use this link for the bookings). Why? Is that because Greyhound has to declare the passenger list to the US CBP or some other US agency ahead of time? Check the passenger travel document ahead of departure? Some other reasons?
Eg, on Sunday February 19, 5 PM PT, one can't book any of the following path before Wednesday February 22.
- Austin to Monterey: no trips within the next 3 days.
- El Paso to Ciudad de Juarez: no trips within the next 3 days.
- San Diego to Tijuana: no trips within the next 3 days.
The last two paths are especially very frequent with ~6 buses available each day.