The U.S. Postal Service (USPS) is a reliable way to send your mail – provided you place the right postage on your envelope. Incorrect postage can result in a delay in shipment or your recipient never receiving the mail. Fortunately, resolving the issue is easy to do. Your mail eventually will get there.
TL;DR (Too Long; Didn't Read)
Either the sender or receiver is required to pay the remaining postage, depending on the circumstances – with the exception of absentee ballots.
If you send a first-class letter or package without enough postage, the post office will try to deliver the mail with a "Postage Due" notice. If the recipient refuses to pay, it will come back to you with a notice of insufficient postage. Alternatively, the post office may return it to you with a notice without trying to deliver it to the recipient.
When you've mistakenly underpaid on a package or letter that you sent through International Priority Mail or Global Express Mail, the USPS website states that it will still deliver your package promptly so as to meet the delivery deadline. However, you will receive notice that the post office has shipped your item, but that you need to pay more postage. For domestic Priority or Express mail with insufficient postage, the post office will contact you about the additional postage and hold the package until you pay.
No Return Address
If there is no return address on your letter or package, the post office will try to send it to the recipient with a "Postage Due" stamp. The recipient will have to pay the additional postage before being able to receive the letter. If the person declines, then it becomes "dead mail." If it's a magazine or other printed matter without sufficient postage, the post office will dispose of it. If it's a package or first-class letter, workers will open the package to see if there's a way to determine the address.
Resolving the Issue
Resolving insufficient postage issues is easy. If the post office returned your letter with an "Insufficient Postage" stamp, you can simply cross off the stamp, add the correct postage and place it back in the mail box. The original stamp used is reusable in this case.
Insufficient postage could present a serious problem when it comes to absentee ballots. Returning the ballot to the sender may mean that there's not enough time to send it again. For this reason, the post office makes an exception and will deliver all absentee ballots, even if they do not have enough postage.