I steal from the federal government on a regular basis.
There are two sides to the post office here: the supply side and the mailing side. Each has it's own cashier. You cannot mail something from the supply side. But somehow, the post office has lent itself to such a “community” environment that you can basically walk over to the supply side, grab any packing material (I usually choose a small media box at $2.49), and take it to the mailing side where apparently it's like crossing the state border on a police chase. The supply people can't get you anymore and the mailing people have no idea if you've already purchased the box or not. And believe me, they won't ask.
I think the confusion actually occurs when you take the box (unpurchased) over to the shipping table in the middle of both sides and start filling out your address on it. The supply people figure you're going to come back and pay for it when you're done filling out an address, and the mailing people figure it's already yours.
Classic case of apathy in our government on the smallest level if you ask me.
No, wait…that would be when I steal stamps from the vending machine in plain view.
Kidding! Kidding! I don't need stamps. I just mail unstamped letters with the return address the same as the sender address so it gets there either way.