I think the envelope was simply the evidence that would have brought Marky Mark back into the picture. DiCaprio's character said they needed to get Dignam back to confirm shit, he got the envelope together and gave it to the good Doctor. I figure she went right past Damon at the funeral because she opened the envelope and knew he would be taken care of.
Envelope = Evidence and contact info for Marky Mark to carry out the final coup de grace.