Mona's green triangle tattoo is a Nazi concentration camp symbol. Specifically, pink triangles were for homosexuals, red for political prisoners, blue for emigrants, black for the catch-all "asocial" prisoners, and green was for criminals. This is foreshadowing, since Mona does become a criminal.
The boyfriend mailing himself in a package to his girlfriend and being killed when someone tries to open the package and stabs the boyfriend in the top of the head mirrors the Velvet Underground song The Gift.
The Gift ,off of the 1968 album White Light/White Heat, includes lyrics about a man named Waldo Jeffers who mails himself to his girlfriend. When his girlfriend has difficulty opening the package a sheet metal cutter is plunged "through the middle of the package, through the masking tape, through the cardboard, through the cushioning, and right through the center of Waldo Jeffers' head."