That's what we're going for in August! My dad is going to be down there, so we're going to go hangout with my mom and keep her company. Any excuse to go to Disney
I think your numbers are off. They do these conventions yearly
and last years convention had 7500 representatives and the year before the numbers were similar so I'd guess it's more in line with those numbers.
Also, this is a convention for NALC, which is the union for the post office. The majority of the people coming for this convention work specifically with the union and are not letter carriers (I'd say probably around 75%) and therefore not that many letter carriers are going to be missing work so mail wont' be impacted
I do think it's important to point out, that they try to stay in union approved hotels, using hotels that offer unions and are in good standing or something like that. I know the hotels on the list that are highly recommended include: Hilton, Sheraton Park, Paradise Pier and The Disney Hotel. Once those are booked, the other hotels they have group rates/blocks for are: Anabella, Anaheim Desert Palms, Best Western Pavillions and Stovalls, Clarion, Doubletree, Embassy Suites, HoJo Plaza, Hyatt Regency Orange County, Portofino, and Red Lion. So if you're thinking of staying at these hotels during this time, try to book early and be prepared for very crowded hotels!
And I also want to point out that these classes/meetings keep the attendees pretty busy during the day so I'd expect them to filter into the parks in the late afternoon/ evenings and for it to get crowded then.