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The carrier catches it. Parcels roll over automated scales and dimension scanners in sorting facilities, and when the measured weight exceeds what the label paid for, the carrier charges the difference. That charge lands on your shipping account, gets collected from the recipient as postage due, or the parcel is returned to you, depending on the carrier and how far off the weight is.
| Carrier situation | What typically happens | Who pays |
|---|---|---|
| Small overage, label bought on your account | Automatic postage adjustment billed to your account | You, often days later |
| Overage with no account on file | Parcel delivered postage-due or held for pickup with payment | The recipient |
| Large overage or wrong service class | Parcel returned to sender or held at the depot | You, plus the delay |
| Flat-rate service (e.g. flat-rate Xpresspost) | Nothing, as long as you are within the weight cap and use the right packaging | Nobody: that is the point of flat rate |
You are not dealing with a clerk eyeballing boxes. Modern sorting facilities run parcels over in-motion scales that weigh each item in a fraction of a second, paired with laser dimension scanners that capture the box's length, width, and height. The system compares all of it against the label data automatically. This is why "close enough" guessing fails at scale: the check is machine-driven, consistent, and applies to every parcel, not just suspicious ones.
There is usually a small tolerance built in, but it is not published as a promise and it varies by carrier, so do not plan around it. A few grams over on a kitchen scale versus a calibrated industrial scale is also a real source of surprises: home scales drift, and carriers round up.
Postage due means the recipient has to pay the shortfall before the carrier hands over the parcel. For a personal gift, that is an embarrassing moment at the door. For a sale, it is worse: buyers did not agree to extra charges, refused postage-due parcels are common, and the parcel then travels back to you at your expense. A one-star review over a few dollars of postage is a bad trade.
The timing makes it sting more. Adjustment charges to your account often arrive days or weeks after the shipment, buried in a statement. If you are a casual seller, that surprise debit can be genuinely confusing until you connect it to the under-weighed parcel.
Sometimes. If you catch the mistake before drop-off, void the label and buy a correct one: most label providers let you void unused labels, sometimes with a small fee or a short refund window. If the parcel is already in the system, you generally cannot upgrade the label mid-transit; the adjustment process takes over. Check whether your label has expired before reprinting, since voided and expired labels are different things with different refund rules.
The cheapest fix is prevention. Weigh the finished, sealed, fully taped parcel, not the item alone: box, cushioning, tape, and the label pouch all count. Round up, not down. And if you ship the same thing repeatedly, write the packed weight on a sticky note on your packing station so you stop re-weighing every time.
Largely, yes, which is why flat-rate options are popular with small sellers. With a flat-rate label, the price is fixed as long as the parcel fits the service's rules: under the weight cap, in acceptable packaging, going to a covered destination. LetterAgent's shipping labels work this way: a Canada Post Xpresspost label for Canada-only addresses, flat rate up to 5 kg. If your packed parcel is within the cap, the exact gram count stops mattering, and the whole postage-due scenario goes away. You still need the destination to be inside Canada, since the label only covers domestic shipments.
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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