Round up, never down
Carriers round to the next whole pound in their own favour. A parcel measured at 4.1 lb is a 5 lb parcel, and pretending otherwise buys a deficiency notice rather than a saving.
Short-paid mail
Postage due
Postage due is what happens when a mailpiece arrives carrying less postage than it needs: USPS marks the piece to show the total deficiency of postage and fees, then collects it from somebody. Which somebody depends on how the postage was bought, and for parcels with an online label the modern answer is that the shipper is billed the difference rather than the recipient being asked for cash at the door.
DMM 604 quotedOnline-label path explainedDispute window cited
The cheapest fix is a scale
Every postage-due story starts with a weight or a dimension that was guessed rather than measured.
The traditional process
The rules for underpaid mail are old, specific and still in force. Under DMM 604 8.1.1, mail of any class received at either the office of mailing or the office of address without enough postage is marked to show the total, rounded off, deficiency of postage and fees.
What happens next depends on how many pieces are involved. An individual piece, or fewer than ten of them, is delivered to the addressee on payment of the charges marked on the mail. Where there are ten or more pieces, the mailer is notified so that the postage charges may be paid before dispatch.
| Case | How USPS handles it |
|---|---|
| Fewer than 10 pieces | Delivered to the addressee on payment of the charges marked on the mail |
| Ten or more pieces | The mailer is notified so the charges can be paid before dispatch |
| First-Class Mail with a return address | Returned to the sender with the reason for nondelivery Returned for Additional Postage |
| Other classes with a return address | Returned to the sender and delivered when the sender pays the total deficient postage |
| No return address | The piece becomes dead mail |
The line that stings is the last one. A short-paid piece with nothing on it to identify the sender has nowhere to go in either direction, which is one more argument for a return address on every carton.
Online labels work differently
Almost nobody buying labels on a screen will ever see a recipient asked for money at the door. USPS runs an Automated Package Verification programme that, in its own words, automatically detects and corrects certain package postage overpayments or underpayments, catching discrepancies during processing and crediting or debiting the shippers directly.
That is a meaningful shift in who bears the risk. The parcel keeps moving, the recipient is not involved, and the adjustment arrives on the shipper's side afterwards. Overpayments run the same machinery in reverse, which is the part people forget when they assume every adjustment is a penalty.
There is a route to argue with one. USPS states that you have 60 days from the date of the notification to file a dispute for a given transaction, with expected responses in 2 to 5 business days and up to 15 days at the outer edge (per usps.com, checked 2026-07-28). Keep the packing photo and the scale reading, because a dispute without evidence is a coin toss.
Our own handling of the same situation is described on the estimate accuracy page: when a carrier re-measures a parcel and bills the difference back to the label, that is a carrier adjustment, and it exists on every shipping platform rather than being peculiar to us.
Why the price moves with the box, not just the scale
Most adjustments come from a wrong weight, and the rest from a box bigger than the one described. The ladder below is a set of live SMKlog quotes on one lane, Newark, NJ 07102 to Atlanta, GA 30303, captured 2026-07-28, where both the weight and the carton grow together the way they do in real packing.
| Weight | Carton | Price | Transit window returned |
|---|---|---|---|
| 0.5 lb | 9 x 6 x 3 in | $8.86 | 8-9 business days |
| 1 lb | 10 x 8 x 4 in | $9.91 | 8-9 business days |
| 2 lb | 12 x 9 x 4 in | $8.48 | 8-9 business days |
| 4 lb | 12 x 10 x 6 in | $8.48 | 8-9 business days |
| 8 lb | 14 x 12 x 8 in | $15.73 | 8-9 business days |
| 16 lb | 16 x 14 x 10 in | $22.62 | 8-9 business days |
That ladder is not a straight line, and the kink is instructive: the 2 lb and 4 lb rows both came in below the 1 lb row, because pricing responds to the whole parcel rather than the number on the scale. Anyone estimating postage from weight alone is guessing at half the inputs, which is precisely how a short payment happens.
The habits that prevent it are dull and effective. Weigh the sealed carton rather than the contents, measure the longest points including any bulge, and enter what you measured instead of what the box was sold as. Our label price guide shows what those inputs are worth in real money before you commit.
Keep the evidence for 60 days
That is the dispute window USPS publishes. A photo of the sealed box on the scale, taken before it left, is the difference between a refunded adjustment and an accepted one.
A return address is not optional
Short-paid mail with no return address becomes dead mail under DMM 604. Two lines of print keep a parcel recoverable in the worst case.
Postage due questions
What does postage due mean?
It means a mailpiece arrived without enough postage. Under DMM 604 8.1.1 the piece is marked to show the total, rounded off, deficiency of postage and fees, and USPS then collects that amount from either the addressee or the sender depending on the class and the circumstances.
Who pays postage due, the sender or the recipient?
On paper mail it can be either. Fewer than ten pieces are delivered to the addressee on payment of the charges marked on the mail, while First-Class mail with a return address goes back to the sender marked Returned for Additional Postage.
Does postage due apply to online shipping labels?
Usually not in that form. USPS runs an Automated Package Verification programme that detects and corrects package postage overpayments and underpayments during processing and credits or debits shippers directly, so the recipient is not asked to pay.
How long do I have to dispute a postage adjustment?
USPS states you have 60 days from the date of the notification to file a dispute for a given transaction, with responses expected in 2 to 5 business days and up to 15 days at the outside, checked 2026-07-28.