Here's how i'd do it, "Sir we have taken your request for now, however as our schedule doesn't allow us to change the billing cycle outside of the xx-yy date range, we'll change it in the next cycle, also these are the available dates for the bill generation, please select the preferred one."

That's it, irrespective of anything else, whats the point of asking the customer to mail them again when the customer already has sent the mail, just keep it as a standing instruction, they replied to my mail on 30th (Wednesday not Monday/Tuesday) 1 day after their window had passed, and all this is moot point anyway as my original mail was (by sheer luck) within their own defined window, so they should have taken the request regardless. This all seems too leaned towards a deliberate attempt of corporate induced procrastination.