International reply coupon. You can buy it at the big post office. The other party can exchange this for stamps locally. SASE is troublesome and needs local reply envelopes and stamps. The usage is to send it together.

Airmail envelope, buy it at the post office. Buy outside, afraid to buy things that don't meet the requirements.

Same as 1

Same as 1.

About seven dollars. When you take the letter to the post office, the other party will weigh it for you and then determine the postage. But generally, the maximum number of stamps you buy is 1, and the envelope may not be enough. If you give the money to the post office, most post offices will stamp it for you, indicating that you have paid the postage.

Finally ... the above steps can only be done by going to the post office.