Ask HN: Is this a snail mail Hack... or just a really bad idea?

This is really a question about how mail routing works, because it's something I really don't understand.

Imagine the following scenario: You are a cheap bastard that wants to hack the mail system to mail a package. So you take your package, put insufficient postage on it, address it such that the "return address" is actually the target shipping address and the "shipping address" is actually your home address. Now what will happen when you drop the thing off at the post office?

The mail system seems as though it should allow a package to be mailed from anywhere regardless of the zip code of the return address, but if the postage is insufficient, it seems as though if they have an established way of dealing with it, then there wouldn't be any way for them to stop people from "hacking" the system like this other than making all returns C.O.D. Is that what they do?

Note: I've never done this, but it is a thought I had a while back, and a recent encounter where I was asked to put someone else's return address on a package I was mailing sparked it again.