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The carrier usually catches it. Parcels pass over automated scales and dimension scanners in sorting plants, and if the actual weight is higher than the label states, the carrier bills the difference to the account that bought the label, sometimes with an extra fee. Understating weight can also delay delivery while the parcel is held for payment.
Modern sorting plants weigh and measure nearly every parcel as it moves along the belt. In-motion scales capture the weight without stopping the parcel, and dimension scanners measure length, width, and height at the same time. The system compares those readings with the weight and size on the label. A mismatch flags the parcel automatically, long before a human ever looks at it. Small differences within the carrier's tolerance may pass unnoticed, but a parcel that is clearly heavier than its label will be caught.
Size matters as much as weight. Many carriers charge by whichever is greater: the actual weight, or the dimensional weight calculated from the box size. A large, light box can therefore cost more than its scale weight suggests, and understating either figure can trigger an adjustment.
| Mistake | What usually happens |
|---|---|
| Label weight lower than actual | Carrier bills the difference to the label buyer's account, sometimes plus an extra fee |
| Label weight higher than actual | Parcel usually delivers normally; you paid for more weight than needed |
| Weight right, box size understated | Dimensional weight adjustment can apply, same as a weight error |
| Weight wrong on a prepaid label | Adjustment billed to the account, or the parcel held until the difference is paid |
The account that bought the label pays, not the recipient, in most cases. If you bought the label through a carrier account or a label service, the adjustment appears on that account. If you are a seller, that cost comes out of your pocket unless your buyer agreed otherwise, which is why accurate weighing matters for anyone shipping regularly. A pattern of understated weights can also draw attention: carriers may audit accounts that show repeated mismatches.
Usually the parcel still moves, and the adjustment is billed after delivery. But some carriers hold a parcel when the difference is large, or when the label account cannot be billed, until the extra amount is paid. A parcel addressed to a business that refuses to pay an adjustment can also be returned. If tracking shows a parcel stuck at a sorting plant with a note about postage or weight, a weight mismatch is one of the first things to check.
Weigh the finished parcel, not just the item: box, padding, tape, and label all add weight. A bathroom scale works for heavier parcels if you weigh yourself holding the parcel and subtract your own weight, and a kitchen scale handles smaller ones. When in doubt, round up to the next weight step rather than down: overpaying slightly costs far less than an adjustment plus an extra fee. Measure the box too, and enter the outer dimensions honestly.
If you get a label through LetterAgent, the same discipline applies. You ask your AI assistant, get a per-label quote, approve the exact price, pay once through Stripe, and receive a label PDF for a Canada Post Xpresspost parcel within Canada, up to 5 kg. Weigh the sealed parcel before you ask for the quote, so the weight on the label matches the parcel the carrier will scan.
Ready to ship it? Get a shipping label with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and get your label.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
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