[mistercow]

Aha! "Unsolicited" was the missing piece in the Google puzzle. It's actually not common law. It's 39 USC § 3009: http://www.law.cornell.edu/uscode/text/39/3009, and was passed in the Postal Reorganization Act of 1970.

I believe this was originally in response to shoe manufacturers mailing people shoes and then invoicing for them if they weren't sent back.

As for whether it applies to non-USPS shipments, I have strong doubts. The law says "mail", and my understanding is that because the USPS is a protected monopoly, non-USPS carriers are explicitly not mail services.