By Kimberly S. Johnson, Globe Correspondent

Think that doing all your shopping on line will exempt you from paying sales taxes? Not so fast.

And if you also think that the amount of tax you should pay is easy for e-businesses to compute, well, things just aren't that simple in the world of cyber-shopping.

There are about 30,000 tax jurisdictions in the United States - states, cities, municipalities - and many of them tax different items at different rates. As e-commerce becomes a driving force on the Web and in the national economy, businesses, state and local governments, and newly formed advisory groups are tackling the issue - amidst a chorus of voices clamoring for a tax-free Internet.

''It's a mixture of confusion and a lot of hype,'' says Jeff Mackie-Mason, a professor for the School of Information at the University of Michigan. ''I don't see why it's any different than mail-order catalogs.''

Arguments in favor of a tax-free Internet hold that the industry is still in its infancy, and, therefore, should receive preferential treatment in order to grow.

But opponents say that cries for ''no Net taxes'' are misguided. These tax advocates say states would lose too much revenue.

But the real issue is not whether to tax or not to tax. It's how to tax - how to apply tax rates and collect taxes for items sold on line and shipped to a physical address. Also at issue is how to tax information bought on line and downloaded directly to a computer.

By law, catalog companies must collect sales tax from buyers who live in states where the companies have a physical presence. So L.L. Bean, headquartered in Maine, must collect Maine sales tax from its buyers who live in that state.

For businesses that exist solely in cyberspace, it's unclear if the same laws apply.

Some companies that have retail stores but also sell through a Web site, such as Staples.com, charge on-line customers sales tax according to the tax regulations of the state where the consumer lives.

''We charge tax where tax is due on a state-by-state basis,'' says Jeffrey Levitan, senior vice president of Staples.com. With 824 stores in 43 states, the company's Web site uses a software program to compute the accurate tax each time a customer places an order.

Other sites have different policies, or find ways around tax laws. For example, bookseller Barnes & Noble must charge sales tax on books sold in its stores in all 50 states, but its Web site, barnesandnoble.com or bn.com - a separate company from the bookstores - charges sales tax only to buyers who live in the three states where the site has a physical presence: New York (its corporate headquarters), New Jersey (its factory), and Virginia (some of its servers).

Sales tax isn't the only tax businesses and consumers need to consider. Even if on-line vendors don't collect the correct sales tax, it is the responsibility of consumers to pay a use tax to their home state.

''Most citizens don't know they have to pay a use tax,'' says Heather Rosenker, executive director of the Advisory Commission on Electronic Commerce, a group of political and business leaders that is charged with delivering recommendations about Internet taxes to Congress by April 21, 2000.

In fact, most citizens probably don't know what a use tax is. According to Rosenker, it's a tax charge for the use of the infrastructure - trains, highways, etc. - needed to get the product you bought to you. States rely on the ''good faith'' of its residents to report their purchases and pay the appropriate tax, she says.

Collecting use tax for large purchases, such as a car, is easy for governments because cars must be registered. But in most cases, it's more difficult for states to collect the tax. And since the commerce clause of the Constitution says that states cannot place ''an undue burden on interstate commerce,'' governments can't force companies to do all of the costly administrative work it takes to track and collect taxes from each consumer.

In cyberspace, where many companies are small start-ups, some say the expectation that an e-business can collect tax is unreasonable.

But states are starting to worry about the burgeoning e-commerce industry, and that they may not be getting their cut.

''This might be a big deal, a serious [loss of money] from local retailers,'' says Charles McLure, a senior fellow at the Hoover Institute at Stanford University.

In the future, perhaps, but right now, the amount of uncollected sales and use tax is estimated at just one-tenth of 1 percent of total US sales - or about $170 million per year.

Regardless of how much revenue is lost, everyone agrees that the current on-line tax system is a mess. There are several committees of politicians, retailers, and economists trying to come up with a solution acceptable to all.

The Internet Tax Freedom Act, passed a year ago, placed a three-year moratorium on taxes on Internet activity. Rosenker's advisory commission has held two meetings and asked for suggestions from the public on how to handle taxation on the Web.

Meanwhile, McLure suggests a simplification of the entire tax system. He says three basic rules are essential to a uniform tax system:

If an item is sold to a business, don't tax it.

If an item is sold to a household, tax it, unless it is a necessary item such as prescription medicine.

Apply tax regardless of the physical location of the seller and buyer, using one nationwide tax rate. This would be especially helpful in cases where the true location of a seller or buyer can't be determined easily.

''Any state that doesn't have a sales tax ... can collect the receipts of its citizens and then give the money back,'' McLure says.

A uniform, national tax code for on-line purchases would require congressional action and approval from every state's Legislature. That could be an uphill battle: Even though Congress has called for an examination of the issue, it has been ''loathe to interfere in state tax law,'' says Joe rosby, national director of state taxation legislation services and policy for the accounting firm Ernst&Young.

Until that time, e-commerce is a tax free-for-all, for both buyers and sellers.

Kimberly Johnson is a researcher for the Globe. Her e-mail address is kjohnson@globe.com.