So for me envelope has a mostly negative connotation, I hail from a generation of instant-gratification and excessive debts and correspondence is almost all digital, paper mail means business, not pleasure. The envelope means money I owe.
This is similarly how I feel about the energy envelope, I understand it's importance and the responsibility but I dread it's very existence. Can't I just ignore it? That's what I do with all those hospital bills...
just kidding. Sometimes.
And just like those dang bills, they all eventually collect their dues.
http://solvecfs.org/wp-content/uploads/2013/06/080505.pdf