If there is an insufficient amount of stamps on an envelope, how long does it take for it to be returned to the sender?

If the First Class Mail envelope weighed over one ounce, then more than a single $.49 Forever stamp would be required to cover postage; two $.49 Forever stamp would be necessary.

The Postal carrier may deliver the letter marked as postage due to the destination address, and the recipient will need to pay the amount due before the Post Office will hand over the letter.

"1.5 Shortpaid Mail—Basic Standards - Mail of any class, including mail indicating special services (except Express Mail, registered mail, and nonmachinable First-Class Mail), that is received at either the office of mailing or office of address without enough postage is marked to show the total (rounded off) deficiency of postage and fees. Individual such pieces (or quantities fewer than 10) are delivered to the addressee on payment of the charges marked on the mail."

The Post Office will send the letter back to the return address shown on the undeliverable envelope because of insufficient postage. If the envelope does not have a return address, the envelope will be treated by the Post Office as dead mail.

"1.9 Undeliverable and Refused Shortpaid or Unpaid Pieces - Mail with insufficient postage that is refused by the addressee (recipient) or otherwise undeliverable is returned to the sender with the reason for nondelivery 'Returned for Additional Postage' if First-Class Mail with a return address so sender may affix the additional postage, cross out the reason for nondelivery, and remail the piece..."

http://pe.usps.com/archive/html/dmmarchive0810/P01...

Amount Of Stamps