A surprising number of shipping invoices in Australia charge for a weight that has nothing to do with what the scale said. That's not an error — it's how freight pricing is designed to work, and not understanding it is one of the most common reasons a "light" shipment turns out to be an expensive one.

The formula carriers actually use

Freight carriers don't bill on actual weight alone. They bill on chargeable weight, which is the greater of the shipment's actual weight and its volumetric (cubic) weight:

chargeable weight = max(actual weight, volume (m³) × cubic conversion factor)

The conversion factor varies by freight type. Industry references put express freight at a factor of 250kg per cubic metre in Australia, with air freight, international courier, general freight and sea freight all using different multipliers again (One World Courier, volumetric weight explainer). In plain terms: a light, bulky carton — think a lampshade, an inflatable, a stack of empty boxes — gets billed as though it weighed 250kg for every cubic metre it occupies, regardless of what it actually weighs on a scale. Under-declaring the size of that carton doesn't lower the price; it just means the carrier reweighs and re-dims it later and the merchant absorbs the difference as a surprise adjustment instead of a known quote.

This is also why accurate packed dimensions matter more for freight-eligible products than for anything else in a catalogue. A parcel that's slightly oversized on a courier network might just get rejected or reweighed; a freight shipment that's under-measured gets a chargeable weight recalculated against a formula the merchant never saw applied.

The other fees that ride along with cubic pricing

Cubic weight rarely arrives alone. On the same TNT Australia domestic surcharge schedule that sets out the residential and tail-lift fees covered in our residential delivery guide, overlength freight is banded by declared dimension: surcharges start around $18 for freight between 1.5–1.99 metres and climb sharply through each band, reaching several hundred dollars for freight in the 5–6 metre range, with anything longer priced case-by-case (TNT Australia surcharges). Dangerous goods handling, remote-area zone loadings, and fuel levies are all separate, additive line items on top of the base cubic-weight calculation — which is exactly why a single "cheap-looking" freight quote can arrive as a materially different invoice once every applicable accessorial has been added.

None of these fees are hidden by the carriers — they're published, exactly as referenced above. What's missing for most merchants is a system that applies all of them before the shipment is booked, using the shipment's actual declared dimensions, instead of leaving the merchant to reconstruct the invoice after the fact and wonder which line item did the damage.

Why "just add a buffer" doesn't fix it

A common workaround is to pad every freight quote with a flat buffer to absorb whatever accessorial fees might apply. This fails in both directions at once: it overcharges the customer on shipments that were never going to attract those fees (which pushes straight into the checkout abandonment problem covered in our checkout accuracy article — extra, unexplained cost is the single most common reason shoppers abandon a cart, cited by 39% of those who do, per Baymard Institute's research (Baymard Institute)), and it still under-charges on the shipments where the actual fees exceed the buffer. A flat percentage markup can't replicate a formula that depends on declared cubic volume, address type, and site accessibility all at once.

What accurate cubic pricing looks like in practice

The fix is to calculate chargeable weight from real, confirmed or high-confidence-estimated dimensions at the point of quote, apply the same overlength banding and accessorial logic the carrier will apply at invoice time, and show the merchant (and where relevant, the customer) the components of that price rather than a single opaque number. That's also the point at which a shipment should be automatically routed to the right network — a load in freight territory shouldn't be quoted by, or accidentally booked through, a parcel courier that was never built to carry it, which is the subject of our guide to Pack Network vs Pack Freight-style managed networks.

How Pack Freight calculates chargeable kg (250 kg/m³), overlength bands, and the quote breakdown is in Cubic Weight and Chargeable Freight. For where packaging decisions interact with cubic pricing — right-sizing a carton reduces both freight cost and dimensional exposure — see the Packaging Optimisation help doc.