Why your parcels cost more than the rate card says
Carriers bill on the space a parcel occupies, not what it weighs. Most shops discover this one invoice at a time.
A rate card is a table of weights and prices. It is also, for most shops, a poor predictor of the invoice. The gap between the two has a name: dimensional weight, or volumetric weight, and it is the single most common reason a shipping budget is wrong.
What carriers actually charge for
A van holds a fixed volume long before it holds a meaningful weight. Fill it with pillows and it leaves the depot full and barely loaded. Carriers therefore bill on whichever is greater: the actual weight, or a figure derived from the parcel's dimensions.
The usual European formula divides length by width by height in centimetres by a divisor, commonly 5000. A 40 by 30 by 20 centimetre box comes out at 4.8 kg of dimensional weight. If the thing inside weighs 900 grams, you are billed as though it weighs 4.8 kg.
| Box, cm | Volume | Dim weight at 5000 | Actual | Billed |
|---|---|---|---|---|
| 40 x 30 x 20 | 24,000 | 4.8 kg | 0.9 kg | 4.8 kg |
| 30 x 20 x 15 | 9,000 | 1.8 kg | 2.0 kg | 2.0 kg |
| 50 x 40 x 30 | 60,000 | 12.0 kg | 3.5 kg | 12.0 kg |
The second row is the only one where the rate card told the truth. That is a packaging outcome, not a negotiating one.
Where the money leaks
- A single box size for everything, chosen because buying one size is cheaper.
- Void fill used to stop movement in a box that was too big to begin with.
- Product dimensions recorded for the website but never for the parcel.
- Multi-item orders packed into one large box when two small ones would bill for less.
The last one surprises people. Two parcels at 1.8 kg dimensional weight frequently beat one at 12 kg, because the divisor punishes volume much harder than the second label costs.
What to do about it
Record real dimensions per product, not per catalogue. Hold three or four box sizes rather than one. Then let the shipping layer compare services using the dimensional weight it can calculate, rather than the weight a human typed in. If the system knows the box, it can tell you which service is genuinely cheaper for that box, which is a different question from which rate card looks cheapest.
The cheapest rate card line and the cheapest shipment are rarely the same line.