When you said in your first post,
... all the product prices in my website are being displayed without VAT.....online users are able to buy products and their total amount is still without VAT.
it sure didn't sound like a single case with a single user, but rather as everything "tax" was suddenly a big mess.
Any ideas how a single case could "slip through" when you're unable to reproduce it under the very same conditions as him? Hardly. And certainly not without knowing all relevant details of your system. Which, in fact, remain completely unknown.