Because a courier charges for the room your parcel takes up in the van, not only what it weighs.
A van fills by volume long before it reaches its weight limit. A courier paid only by weight would carry a van full of cushions for almost nothing.
The sum
Length times width times height in centimetres, divided by 5000, gives a weight in kilograms. You are billed the greater of that and the real weight.
A 50 × 40 × 40 cm box: 50 × 40 × 40 = 80,000 ÷ 5000 = 16 kg. A duvet in that box weighs maybe 2 kg. You pay for 16.
That is not a penalty. It is the honest cost of the space.
What this hits hardest
- Lampshades, cushions, soft toys
- Anything boxed: shoes in the original box, boxed toys, appliances
- Framed prints and canvases
- Baskets, planters, anything hollow
- Bubble wrap, when there is more of it than item
If what you sell is on that list, this single sum decides whether your delivery pricing works.
How to shrink it
Cut the longest dimension first. It multiplies into the total, so the biggest side is where the saving is. Dropping 50 cm to 40 cm on that example takes 16 kg to 12.8 kg.
Use a box that fits. A 20 cm vase in a 45 cm box is a 45 cm parcel. Padding is not free.
Flatten and roll what can be. Fabric, prints, soft goods.
Send the item without its display box where the buyer does not need it — but say so in the listing, because for sneakers and collectibles the box is part of what they bought.
For sellers, the money part
Measure a packed parcel before you set your delivery price, not after your first sale. One measured box tells you more than any estimate.
Enter the real figures. If a parcel is bigger than declared, the courier charges the difference afterwards and it comes off your balance as a later deduction. Guessing small defers the cost, it does not avoid it.
Consider a separate delivery option for the bulky things. One average rate across a range that includes earrings and lampshades overcharges the earrings and loses money on the lampshades. What is a delivery profile?
For buyers
When delivery on something light looks wrong, this is usually why — and it is the courier's arithmetic rather than the seller marking it up.
Pickup avoids it entirely, which is why it is common for bulky things. What is a pickup point?
More: what is chargeable weight, why is delivery so expensive, how is weight calculated.