I think part of the misunderstanding here is that people outside of the UK cannot understand that "inquiry" to us means a formal, organised investigation normally held by a group of people.
Making it highly unlikely that anyone on your side of the Atlantic would ever say "Thanks for the inquiry" -- especially not if they were the ones fielding the questions. Conversely, a "public enquiry" would be, uhm, a man-in-the-street interview?
"Enquiry" to an American looks British. "Inquiry" to a Brit looks wrong. Which of the two is more likely to cost you sales? ;)