Beginning Friday, the majority of U.S. consumers will have to pay sales tax on their Amazon.com purchases. That’s a milestone of sorts for the retail giant, which has spent years trying to avoid collecting the tax, a leg up over brick-and-mortar retailers. Laws that take effect in Massachusetts, Connecticut and Wisconsin Friday mean about 163 million Americans in 16 states will have to pay tax on their diapers, books and other goods on Amazon. Many state laws affecting sales tax target Amazon because of its relationship with affiliates who send Web traffic its way in exchange for commissions. The states are relying on a 1992 U.S. Supreme Court decision that permits them to require retailers to collect sales tax only when the retailer has operations in the state. Amazon continues to fight sales taxes, but its position has softened.