Bird,
"Media mail" is a new term for what used to be called "4th Class Book Rate." Book Rate was/is a special, cheaper rate for books, especially important for educational institutions and libraries.
I guess they expanded this rate to include any type of learning media---books, computer disks and files, etc. Hypothetically the special lower rate is meant to encourage "expansion of knowledge." Am not sure if that will be the result with your XP, but let's hope so! These days even the "media mail" rates are not that much cheaper than 1st class. But yes, it is the slowest teansit possible, up to 3-4 weeks. As always: "talk walks, money rides."