Leslie Andrews does most of her shopping online. "I wasn't aware of that at all. So it's our responsibility to fill out a form? No I didn't know that."

And chances are, if you shop online, you didn't know either. It's called the consumer use tax and when you buy merchandise online or through a catalog and the seller does not collect sales tax, by law, you must file and pay a use tax on that merchandise.

Erin Schablik says she tries to avoid the sales tax. "I've made sure that I don't buy things that are in Tennessee, because 9.25% is kind of hefty. I think that's bogus, I'm not gonna do that."

According to the Tennessee Department of Revenue, most people don't. Since January 1, 2006, 1,918 consumer use tax returns have been filed. "I'd like to know how much they enforce that," says Andrews.

That's the problem. Matt Murray with the Economics Department at University of Tennessee says it's an unenforceable tax, and there's no way the state can fully enforce it. "Even if the information were available it would be very costly for them to go out and assess tax against you for all of the purchases you made, a CD all the way up to a major appliance."

This happens mainly with companies without a presence in the Volunteer state. "You can go to Lands' End online and you can buy from Lands' End and not pay sales tax to Lands' End. You are still responsible to pay the sales tax to the state of Tennessee," says Murray.

And for states like Tennessee who rely heavily on the sales tax, getting those returns in is very important. "If they don't get that revenue through the payment of the tax, in this case what they have to do is find the revenue elsewhere." says Murray.

Which he says could turn into a higher sales tax rate.

If a company has a store, factory or warehouse in Tennessee, they will charge you sales tax for purchases. Tennessee also shares tax information with other states.

So, if you make a purchase outside the state, and you're not charged a sales tax, you might get a letter in the mail that says you owe money.