The transit time by air express is faster than others, which is suitable for urgent orders.
Normally, air express calculates the weight on actual weight or volumetric weight.
For example: one box is 20KGS, but the box size is 50*50*50cm, the express couriers will calculated the shipping fee based on 25KGS=50*50*50cm/5000.
Shipping time: 3-5 working days to most Amazon warehouse, but it may be delayed due to the peak season or other situation.
Customs clearance: Amazon is not responsible for customs clearance. The express couriers take this responsibility, but customs clearance problems still may occur, the declare value on the parcels to U.S.A shall be under USD800 if you want to avoid customs problems.