Updated
Carriers can adjust the price you paid for a label, and add penalty fees for inaccuracy, after the label has been bought and the parcel has shipped. The adjustment arrives weeks later on the invoice, by which point nobody connects it to a specific order.
The magnitude surprises people. At high volume it can run to a meaningful share of monthly spend, and because it lands after the fact it never appears in the number your shipping platform showed you at purchase.
Surcharges that change between the rate and the ship. Carriers apply surcharges for fuel, peak season, oversized packages and residential delivery. Fuel in particular changes frequently. If you buy a label and the parcel does not physically move for a day or two, the surcharge that applies on collection can differ from the one priced into the label.
Weight and dimensions that do not match the parcel. This is the more common of the two and the more expensive. Carriers re-measure. When what they measure does not match what you declared, they re-rate the shipment and add a fee on top of the corrected rate.
Using USPS as the worked example, from their published price list as of the July 2026 change:
These sit on top of the corrected rate, not instead of it. So an inaccurate dimension costs you the difference in postage and then the fee.
The pattern I see most often is an operation that uses one set of dimensions for everything and only varies the weight. Under a cubic foot that mostly does not matter. Above it, dimensional weight applies and every inaccuracy compounds.
Polybags and bubble mailers deserve particular attention, because their packed dimensions are easy to underestimate and one cubic foot is the threshold where the dimension fees begin.
Pull your most recent carrier invoice and compare what you were actually charged against what your shipping platform predicted at purchase. If the two agree, you are fine and you can stop here.
If they do not, the gap is what this post is about. Reading your actual carrier bill in ShipStation walks through pulling those records so the adjustments appear as their own line items rather than being folded into a total.
If you find a fee category you cannot account for, it is worth asking your carrier representative to itemise it.
Ship promptly after buying the label, so the surcharge you were quoted is the surcharge that applies. Validate addresses, because address correction is its own fee. And get the dimensions right per order rather than per catalogue.
That last one is the one that actually moves the number, and it is the one that is hard to do by hand across a varied catalogue. Computing the package at import rather than assuming it is the logic we build alongside ShipStation.
Knowing how a carrier prices a package is useful. The harder problem is making the right packing and rate decision on every order, when the correct answer changes with the contents, the destination and the rate card.
String builds and maintains that logic inside existing ShipStation accounts, for operations shipping roughly 6,000 orders a month or more. Below that, ShipStation's native automation and accurate product dimensions will get you most of the way there.
Eight patterns that show up when an operation needs custom shipping logic
