There is no postal code in Britain.

2. In China, there is a box with postal code in the upper left corner of the envelope. Shouldn't foreign countries put the postal code in front of the country?

be the same as the above

3. How much are two international reply stamps? Where can I buy 22× 1 1 cm envelopes?

Just ask the post office.

How many days will it take to send the letter, and how many days will it take him to send it again?

It took nearly three months to go back and forth.

He is a star and I am a fan. Can I call him dear or my dearest?

of course

6. Can I use your xx or love xx when signing at last?

can

7. Is it necessary to say "Best wishes" at the end? Can you exchange it for something else?

That varies from person to person and doesn't have to be written.

8. Can you write something like I love you in your letter?

Tom's fan metropolis said.

9. What can you say except that he is handsome and plays Draco Malfoy well?

For example, "I like your ordinary mentality, I also like fishing, you are a competent boyfriend, so let's introduce China" and so on.

10. If there are many grammatical mistakes in the letter, will he understand and mind?

As far as I know, those actors don't read letters very much. Basically, they will cut out the reply address on the envelope you sent and send you a printed reply or signed photo. But if you are lucky, he will read the letter and reply in person, but it is unlikely. ...