one other thing to note is
Alamo and National are the same company (divisions of the same company) and late at night one of the counters may be closed so you will have to go to the other.
This happended to me with National and I did not know, had to walk out to the car area to find out what to do.
Also if you type of car is not available in the specific Alamo section, you can take the same type of car from the National section (or vice versa)