Going online to buy the latest bestseller or those photos from summer vacation may be tax free for most people today, but it won’t last forever.

Come this fall, 13 states will start encouraging – though not demanding – that online businesses collect sales taxes just as Main Street stores are required to do, and more states are considering joining the effort.

Right now, buyers are expected to pay sales taxes on Internet purchases themselves directly to the state when they pay their income taxes. But it’s not widely enforced, and states say it costs them upwards of $15 billion a year in lost revenues, collectively.

“Taxes that it was difficult to collect before will now be collected. And consumers will pay that,” said David Quam at the National Governors Assn., helping lead the five-year effort that brought together state revenue officials, legislators and business leaders.

The question of taxing Internet sales has been in limbo since the dawn of the computer era, when the only issue was catalog sales across state lines.

A 1992 U.S. Supreme Court ruling forbids states from forcing a business to collect their sales taxes unless the company has a physical presence in the affected state. The court noted the dizzying array of tax jurisdictions and widely varying definitions of taxable goods, such as fast food versus groceries.

Organizers of the states’ effort, known as the Streamlined Sales Tax Project, sought to unify tax rules and definitions among the states. They hope to persuade federal lawmakers to pass a new law to overcome the Supreme Court ruling and allow states to take the next step – demanding online companies levy the taxes.

To be accepted as part of the project, a state must change its tax laws to match up with the others. So far, 13 states have come far enough to be part of the project. Five more are approved to join within the next few years, and others have made partial steps.

The states are offering businesses a promise – and an implicit threat. If businesses register and start collecting taxes this year, they’re given a yearlong amnesty from the possibility that states may seek back taxes for online purchases.

“I wouldn’t be surprised if the biggest effect comes at the end of the 12 months when businesses say, ‘I better take this opportunity while I can,'” said Diane Hardt, Wisconsin’s tax administrator and a co-chair of the project.

The 13 states are Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Nebraska, New Jersey, North Carolina, North Dakota, Oklahoma, South Dakota and West Virginia.