Heres a simple step you can take if you need to mail something but have no stamp.  Simply switch the two addresses so that whoever your sending the mail to is in upper left hand corner. This is usually the return address area.  Now just put your address in the middle of the envelope and stick it in your nearerst corner mailbox.
Because there is no stamp, your letter will be returned to sender. Which is actually where you want it to go!

Affording is irrelevant.  When offered a free sandwich or the same sandwich for .40 cents, why pay?  Also, I have never done this.  I just find it interesting that the government leaves an exploitable hole like this for so long.  Further reading on this can be found in the book "100 Things You're not Supposed to Know".

FYI, it is only a federal offense if you take the mail from the "custody" of the post office.  If it is in your mailbox and you open it, it is not a federal offense.

To scam the post office is a crime also

You do not want the postal inspection service all up in your grill. They're like a gaggle of mall security who have been given actual authority and are bored as hell.

and then they go postal!

That's a federal felony.  Now get out of the car and give the beer back to the clerk.

Just buy the fucking stamp.