You can't use "rear" because it has the sense of behind what is in front.  It is usually used in 3-dimensional situation.

You could say "the reverse (side)" of the envelope but this is unusual and formal.  "back" is definitely the best choice.

back can also be used in 3D situation e.g. the back of the room, the back of the car.  Rear is possible in these situations too, and again is more formal than back.  "Back" is the normal choice in many situations so if you are not sure, back is usually the safer guess.