Most carriers calculate shipping costs based on the actual weight of the parcel.
Actual weight refers to the physical weight of the package as measured on a scale.
During fulfillment, the actual weight of each shipment is calculated and used to determine the shipping cost. The shipping costs are divided by weight bucket for most carriers, and this information can be found in the contract under "Delivery price."
Some carriers, however, use volumetric weight, which applies when the volumetric weight exceeds the actual weight. This method ensures that large but light packages are charged appropriately for the space they occupy during transit.
Volumetric weight is calculated based on the volume of the box rather than its physical weight. The formula used is: length (cm) x width (cm) x height (cm) x conversion factor, and each carrier has a different conversion factor to transform volume into billable weight.
Detailed information about this can be found in the contract under "Delivery prices" in the "Note" section. Additionally, the tables in the contract are color-coded to distinguish between charges based on volumetric weight and actual weight.