HI there!
GREAT question! We don't anticipate that the USPS will start charging for the return of First-Class Mail if it is undeliverable as addressed. However, the USPS does charge for the return of Standard Mail - when the mailer uses an endorsement on their mailpiece to indicate as such.
For example, if you are sending out newsletters or brochures as Standard Mail, and you have "address service requested" or "return service requested" printed on the envelopes, then, if a recipient has filed a change of address with the USPS 13 months or more ago, the mailpiece would be returned. You'd need to pay for that return.
Elizabeth
