Furke,
Don't panic. I see these immigration things panics us a lot. If you have done already what you were supposed to based on the very letter, then this is just an oversight which seem to happen a lot in these offices. Similar thing happened to me, they asked for something that I had already submitted.

All you need to do is call them and smack 'that chinese lady' heck out of her on what else she needs. And she will say 'no, you all set, just ignore the letter'. And you can rest in peace.

(of course you need to do your DD, point is, if you believe you have done everything then just a call is in order).