Maybe the team even forced him to help get rid of Garrett's (Yani Gellman) body in return for Ali's. It sounded like there was a struggle before Aria stabbed someone, so if Jason's wound we saw in Season 3, Episode 14: “She’s Better Now”is from Aria's screwdriver, maybe he actually was trying to stop her from being killed.
2. "A" Team messages. Of course, it's also possible Jason is part of the "A" team, in which case maybe Lucas was just an errand boy between members. Seems a bit risky to let him know who they are, though. Not really the normal "A" team style.
3. Something else the "A" team threatened him for. It's just as possible that Jason is the victim here. Maybe the "A" team threatened Jason into giving them something they needed, like the N.A.T. club photos, or a thumb drive or disk with N.A.T videos.
What do you think was in the envelope? Let us know in the comments below!