Why a big light box costs more than a small heavy one

Carriers calculate volumetric weight: length × width × height ÷ 5000. You pay for whichever is higher — the actual weight or the volumetric one.

A 60 × 40 × 40 box weighs 6 kg but counts as 19. That's double the price.

What to do: pick a box that fits the item, not "the one you had lying around," and don't pad empty space just to fill it.