If an online retailer has a physical presence in a particular state, such as a store, business office, or warehouse, it must collect sales tax from customers in that state. If a business does not have a physical presence in a state, it is not required to collect sales tax for sales into that state. This rule is derived from a 1992 Supreme Court decision which held that mail-order merchants did not need to collect sales taxes for sales into states where they did not have a physical presence.