Large, lightweight packages are charged at their volumetric weight, i.e. for the space that they take up rather than the amount that they weigh. This applies to all shipments sent. 

Generally airfreight works on a ratio of 6:1 in other words 6 cubic meters is chargeable at 1 metric ton. 

To calculate the volumetric weight for yourself, multiply the length x width x height in centimeters and divide the total by 6000 (cubic centimeters) and you will obtain the volumetric weight (in kilos). If this is greater than the actual weight, you will be charged for the volumetric weight, if it is less then you will be charged for the actual weight. 

In other words, if you can avoid shipping fresh air you will save money.