Hi Faisal,
Unless you are living in the USA, you won't be using the US Postal rates or anything from the US postal service.
You will be using the postal service from your own country to ship the package and, if it is going outside of your own country, then the buyer's country postal service will end up delivering the package to them.
So if you are in South Africa and you are sending a package to me (in the USA) you will use the South African postal service rates to the USA to determine how much you will charge me.
When you ship the package you will put South African Postage on it and give it to the SA postal service. When it arrive in the USA, then the US postal service will take the package and route it to me.
The same thing will happen if you send something to Australia or to Hong Kong or to any other country.