Sales Tax: $0. Online shoppers have gotten used to seeing that line on checkout screens before they click "purchase." But a case before the Supreme Court could change that. At issue is a rule stemming from two, decades-old Supreme Court cases: If a business is shipping to a state where it doesn't have an office, warehouse or other physical presence, it doesn't have to collect the state's sales tax. That means large retailers such as Apple, Macy's, Target and Walmart, which have brick-and-mortar stores nationwide, generally collect sales tax from customers who buy from them online. But other online sellers,...