You definitely don't want to use "Dear customer who inquired" this is much too impersonal, it sounds as if no thought was put into the communication that is about to follow. Your situation is a little tricky. I would suggest, Thank you for your inquiry. This is not a salutation as such but I think it would work in this case. Avoid these: To Whom it May Concern, Dear Madam/Sir, etc....these are outdated for the most part and again, they signal that little effort was expended in the response.

Dear customer is enough.