Quote:
I've been tempted to send some mail to myself with all kinds of underpayment postage on it, and see if it gets delivered or not.
I decided to try one isolated example and here's the result. I got the cover delivered to me in one business day with payment by an old non-denominated 34-cent postage stamp ... no special markings or questions asked, even though the cover was underpaid by 10 cents.

One curious thing is that I know from experience that the mailbox where I sent the cover would have normally received a cancellation from the Brockton, MA postal facility, however, in this case you'll note it went through Boston. Possibly the underpayment resulted in the cover being redirected before delivery. No delay on my part as the recipient, though.