You can send the Writers Mystery Bag from the Free Gifts page. There are also automated scripts out there which you can use to help send the bags, such as the Massive Sender(MWAddon) or Gift Blaster(Spockholm).
Upon opening 20 bags for the day, the Writers Mystery Bags will automatically be converted into the Blue Mystery Bags.
The Special Loot you can receive from the Writers Mystery Bags is the Biographer (133/179). It will randomly drop when opening the bags, along with other great stuffs such as the +5 Attack, Defense, Stamina, Energy, Skill Points and Stamina Boost.