Thank you for reaching out on Italiapass!
It is advantageous to book online in advance as you can get a cheaper rate for your tickets rather than buying them at the station upon arrival. A trip from FCO airport - Naples involves a train change at Rome Termini station. Your trip from the airport to Rome Termini will likely take place on a Leonardo express train. A Leonardo express ticket is special as the ticket is valid for the whole day of travel - so no need to worry about delays getting to Rome Termini. For the portion of your trip from Rome Termini - Naples, you will likely be taking a high speed. High speed train tickets are only valid for the time they were booked, unless an exchangeable ticket is purchased. To exchange the time of your ticket due to circumstances like flight delays, you must ensure to contact us at least 2 hours before the train's original departure time.
You can contact us at 1 877 375 7245 or firstname.lastname@example.org for ticket changes as well as any general inquiries about rail travel in Italy!