If you live in California and have made on-line purchases through Amazon, it's a pretty good bet that you haven't paid a tax that you owe. It's called the use tax.
Amazon and many other online retailers don't collect tax on its sales. The state says Californians are supposed to pay the tax on their returns, but few people do. That's touched off a fight in California (and a number of other states with similar problems) over whether online retailers can be forced to pay the tax.