How to inform the editorial office that a special discount has been received? I just submitted an article in a special issue. This article is based on a talk given in a conference, and we received a discount. So after I submitted the article, the website told me to contact the editorial office about the discount. How shall I write the letter? It is the first time for me to do this kind of thing, please suggest some sample letters. 

I also want to point it out that in the website of the special issue, it is clearly stated that all papers in the issue will be published free of charge. Does this mean that it is not necessary to contact the editorial office? If so, why is it that after the submission, I still got the suggestion to contact the office? 

Thank you!
From the details given in the question it's a bit hard to tell what exactly happened. Still, my read is that the journal is accepting papers for both the special issue and regular issues. You're asked to contact the editorial office to identify your submission as part of the special issue, since otherwise they wouldn't know how to tell one from another. So you write something similar to what you wrote in the question: I'm submitting this article that's part of this special issue, and I'm told there's a discount applicable / free.

On a side note, it's unusual to me that you're asked about payment upon submission, since usually payment only happens upon acceptance (and then only for open access articles). You might want to check if the journal is a genuine one.