A few weeks ago I wondered if you could "beat" postage by sending something with no postage, and the sender/arrival address being the same. Well, pseudo-ironically, this was tested by accident. Someone, who I won't mention *cough*Alan*cough* sent me a New Years card. But, didn't include enough postage. So, my postman put an envelope in my mailbox asking me to put $.17 in it.

So, I guess that's what happens.