I am Aliexpress Top Brand seller, we're looking for partners.

I need an answer on how to choose fast shipping method
Look at the different methods and see which one has the least delivery days. But the fastest will be expensive. Most people choose e-packet with free shipping.

how to guarantee your benefit?
He can't guarantee the services of a third party shipping company. Only the shipping company can do that.

Unless you are talking about his products. In that case Aliexpress has a 45 day customer protection and refund guarantee covering ALL sellers.