“Don't push the envelope. Shove the envelope.”

Source: Quantum Reality - Beyond The New Physics, Chapter 9, Four Quantum Realities, p. 171 ( See also: Principle of locality)

St. 4
Dejection: An Ode (1802)

Alternate translation: Ultimately, it may be our name on the envelope, but someone else (God) is the one who wrote the message.
Pu La had a penchant for ending his humourous works with thought-provoking punchlines. In this quote from his work Post Office, he sums up on how life is a lot like the the letters that pass through the post office.
From his various literature

Thought
One Minute Wisdom (1989)

Interview by David Brancaccio, NOW (PBS) (7 October 2005) http://www.pbs.org/now/arts/vonnegut.html
Various interviews
Context: [When Vonnegut tells his wife he's going out to buy an envelope] Oh, she says, well, you're not a poor man. You know, why don't you go online and buy a hundred envelopes and put them in the closet? And so I pretend not to hear her. And go out to get an envelope because I'm going to have a hell of a good time in the process of buying one envelope. I meet a lot of people. And, see some great looking babes. And a fire engine goes by. And I give them the thumbs up. And, and ask a woman what kind of dog that is. And, and I don't know. The moral of the story is, is we're here on Earth to fart around. And, of course, the computers will do us out of that. And, what the computer people don't realize, or they don't care, is we're dancing animals. You know, we love to move around. And, we're not supposed to dance at all anymore.

Source: The Wind-Up Bird Chronicle

Jokes and their Relation to the Cognitive Unconscious (1980)
Context: All intelligent persons also possess some larger-scale frame-systems whose members seemed at first impossibly different — like water with electricity, or poetry with music. Yet many such analogies — along with the knowledge of how to apply them — are among our most powerful tools of thought. They explain our ability sometimes to see one thing — or idea — as though it were another, and thus to apply knowledge and experience gathered in one domain to solve problems in another. It is thus that we transfer knowledge via the paradigms of Science. We learn to see gases and fluids as particles, particles as waves, and waves as envelopes of growing spheres.

As quoted by Lisa Simeone, Evolution, Not Revolution: Son of the Late Shah Campaigns for Self-Determination in Iran http://www.npr.org/programs/watc/features/2002/jan/shah/020119.shah.html, NPR, Jan 19, 2002.
Interviews, 2001-2002