Recognizing they don't have the right to tax interstate commerce, 40 states are trying a different angle; they're setting up a voluntary system in which states that sign up will ask businesses to opt into taxing their customers. That arguably isn't the hard part. At least as tough will be creating a computer system that will determine the proper sales tax for a given online transaction. Not only do the states have different tax rates, they tax different types of products differently and make odd exemptions and holiday periods.