When you are doing "mail-merge" printing of this quantity, those ARE printed ahead of time by print houses that have the capacity to do what is known as "personalized" printing jobs. They look like someone ran it off especially for you on their printer but in reality it was mass printed as a shell and the printing press is programmed to drop the names onto the shell saving time and money. The customer, in this case Disney, will then wait until all items are complete and drop ship to save money on postage charges.
In other cases, companies order these shells by the 10's of thousands and then laser print the address on for distribution on an as needed basis. Again, these look like individually printed letters and marketing materials but they are not. Given the volume Disney would need to print in a day, it would be impossible for them to individually laser print trip planner packets. They most likely have a shell that states "standard benefits" and spaces for customization.
Magazines are printed anywhere from 1-3 months ahead of time with the exceptions of the national weeklies which are drop printed at satellite plants throughout the country. About half the magazine -- that stuff which is not time sensitive -- is printed ahead of time with the rest being dropped in at the last minute.