You don't remember correctly: VAT has never been added after you inserted the credit card number. It's added or not, depending on your country, in the page that asks for your address, and that comes before the payment page.
The only difference having made a purchase or not in the past on the same pc, is that you won't have to reinsert all the data again, because fields will be prefilled.
This is not an Addon Manager feature, it's a standard Esellerate feature, and the .dll that does it hasn't changed since 2007