Dear xx,
Thank for your letter.
You say "Thanks" to me. I thinks i did what i should do. We are friends. So i'm very glade to help you. And we should help each other as friends. So if i have some trouble, you will help me. Right? i hope in the future no matter who is in trouble, we will give our hand. (中间可以再描述一些过程,和你的愿望)
Best wish.
Yours,
xxx
追问老兄,这个好像有点Chinglish。。。