In episode 19, Tom Keen told Elizabeth Keen to open a box at a bank using the key she found under the lamp.
My (unofficial) prediction is that the envelope contained "orders" of sorts from Raymond Reddington directing Tom to insert himself into Lizzie's life in order to watch over her, keep her safe, keep her happy, or whatever the case may be.
Furthermore, in case you didn't notice, the envelope had a symbol printed on the outside of it which was identical to the symbol carved into the box that Lizzie found (under the floorboards) which contained Tom Keen's passports, cash, firearm, etc:
Ain't that somethin?
And for the record, I still wouldn't completely rule out my (latest) theory regarding who Tom Keen works for.