Why the box, not just the weight
A courier's van fills up by space long before it fills up by weight, so they don't want you posting a beach ball for the price of a brick. They work out a "volumetric" weight from the box size, length times width times height divided by a fixed number, and bill you on whichever is bigger, the real weight or that. The standard number is 5000 when you measure in centimetres and kilograms, 6000 on some express and air services, and 139 for inches and pounds in the US. So a big light parcel is billed as if it were heavy, and the fix is almost always a smaller box: cut the empty space and the bill drops to the real weight. Parcelwise also adds up the length plus girth, because past a certain size couriers tack on a surcharge or won't take the parcel at all. These are planning figures: confirm the divisor, the rounding and the size limits with your own courier before you book.