When you purchase freight through Shipmondo's freight agreement with UPS, you need to be aware of the dimensional weight of your shipment.
Volumetric weight (dimensional weight) describes how much space a specific parcel takes up.
Whenever dimensional weight of the parcel is higher than the physical weight, you will be charged based on the dimensional weight.
The dimensional weight is calculated as follows:
Dimensional weight = length x width x height (all in cm) / Z
Z is a determinant that the carrier sets and at UPS this determinant is Z = 5000.
Round any fraction up to the next half kilogram.
UPS' invoicing is based on whichever value is the highest between the physical and the volumetric weight of the parcel.
20 x 30 x 40 / 5000 = 4,8 kg
Being higher than the physical weight, in this case, it is the volumetric weight of 4,8 kg that would be the basis for the pricing of the shipment.
Fint more info about dimensional weight at UPS here.