@the LFS in subject, there are multiple ways to create good online perception without letting a misunderstanding or the odd dispute impact your online presence, even though the complaint is public.
If you respond negatively, emotionally, or defensively, you are doing more damage to your reputation as a business than some guy mouthing off about GST. Get someone to manage your posts from a PR perspective. This instance could have been good PR if handled correctly.
Have a strategy to deal with negative feedback that shows you are responsive, positive and helpful.