They either use USPS or UPS for FSSS, though anything that isn't FSSS is UPS.
If the entire order can be shipped by media mail, it will be, otherwise, it'll go UPS for FSSS. I ordered Trauma Center 2 + PS3 controller charging base, and it was shipped via UPS for FSSS. Orders with all games, all movies, or some combination of the above items, they all went USPS Media Mail.