15kg-30kg is not heavy.
Don't you know UPS, DHL or TNT can give you 50% discount?
In our website, we only charge customer 50% of the standard air express shipping fee. If UPS, DHL or TNT give us no discount, then all our customers will stop at checking out page!
Yes, there are some other private companies can do lower rate. You can select shipping to the airport of your city by air and pick up by yourself. That's much cheaper than door to door express service.
Door to door express service is very convenient but when parcel is heavy, people usually use shipping to ariport by air service.
I am in shenzhen, China. I don't know the shipping companies in Shanghai or Guangzhou.
But who are you buying from? The seller must know the shipping companies that can do shipping to airport. That's not a secret in international trading business.
Good luck!