Express needs to compare the volume weight and the actual weight, the calculation method 

of the volume weight is: length * width * height /6000 in centimeters (than DHL FEDEX

 and other cost-effective), that one is large according to that one. For example:

 box size: 50*60*50/6000=25KG, the real weight is: 20KG, then charge according to 

25KG, if the same size, the real weight is 30KG, then charge according to 30KG, 

multiple boxes, just add up the volume weight of multiple boxes