Finding the official websites for Roman attractions is...interesting. I think that I just reserved a spot for the catacombs on Tuesday, June 25th at 10:00 AM (reserved on the website https://www.catacombesancallisto.it/) and received an email confirmation, however they are saying you pay for your reservation at the ticket office when you actually show up.
Is this legit? How early ahead of my reservation should I be there to pay? Seems like they'd want to take your money in advance and not have to deal with people who made reservations paying while walk-ups try to get in as well.
But it's Italy and nothing is either efficient or logical.