NEW YORK (Fortune) -- One of the great things about shopping on sites like Amazon has been not having to pay a dime in taxes (or shipping, if you've spent enough). Since the dawn of Web commerce, the rule was that as long as a retailer didn't have a physical presence in the shopper's state, the company didn't collect a sales tax. Well, it was fun while it lasted. Starting June 1, Amazon (AMZN, For