I think you might want to consider why the customer said fuck you here. You're "can't replicate it" response is terrible customer service, and it seems commons sense to expect that they'd be frustrated. Your response of "can't replicate it" feels like a fuck you, if not in so many words.
Also, giving advice based on a single experience rather than several repeated successful outcomes seems like a bad practice for helping other entrepreneurs, and they might not read all the way to the bottom to discover that data point.