The Fixer is received after completing Offered and Lost if Hawke decides that the Qunari bodies should be burnt. Another letter by the same name is obtained when quest was completed to the satisfaction of anonymous principal.
Before the quest is accepted...
I understand that you have certain experience with the disposal of inconvenient evidence. (Like Qunari, perhaps?) I am in need of such skills right now. My men need some things disposed of—formerly living things. You can find them in Darktown, Lowtown, and the Docks. A small effort on your part will reap a large reward.
After all bodies were disposed...
I understand you disposed of the "trash" my men left lying about the city and wilderness. They're so messy, and you did a superb job of cleaning up. Payment is enclosed.