>>1874492
>If secret letter clue farming comes in later,
You can get them fairly easily. Don't mind the duration of the run, only used picrel because it's the only picture i currently have in my hand. But you can get multiples of them within each run, one time i did a sub 1 min escort mission and got rewarded 2 clue letters, other time i got clue letter as bonus reward. Probably dropped by some elite or something. Game needs some sort of drop table. Bottom line if you are playing the game you will accrue enough letters to keep collecting stuff. Casuals may have to rely on quest rewards, dailies and BP though. That or buying up unit for ~$50 each.