International air freight involves two types of weights.
1) Actual Weight
2) Volume/Dimensional Weight –
For international air freight, with any company they use whichever is bigger (dimensional or actual). Dimensional is usually explained to most customers by considering one who wants to ship 100 kg of cotton wool. While this is just 100 kg, it may occupy the space of a whole plane, and you will have to pay for the space it occupies, in this case the whole plane. They don’t say because it just 100 kg, that you pay only 100 kg.
To calculate the dimensional/volume weight of a shipment, please multiply the length x width x height (all in inches) and divide the result by 139. This gives the volume weight. If this is more that the actual weight of the shipment, the volume weight will be used in calculating the cost of shipping by air.
Other companies, FedEx etc also have this. Go to Fedex Dimensional Weight and select Calculating Dimensional weights.
For UPS, go to UPS Dimensional Weight and scroll down to calculating dimensional weights for international shipments.
As a check, when you receive each of the packages. Do this. Measure the length, breadth and height (in inches). multiply these to get the Volume and divide it by 166 (same formula used by Fedex/UPS and American AirSea Cargo, and you will get the exact values you have on the invoices. Now, it is not all the time we run into dimensional weight, say about 5% of the time.