In Illinois, shopping on Amazon used to feel like it came with a discount: no sales tax charged at checkout. That ended three years ago, when the Seattle e-commerce behemoth opened its first distribution centers in the state, gaining an obligation to collect taxes like any bricks-and-mortar retailer.

Shoppers could end up paying sales tax at many more online retailers if the U.S. Supreme Court sides with South Dakota in a case being heard next week.

Under a 2016 law, South Dakota requires any retailer that conducts at least 200 transactions or has at least $100,000 in annual sales within its borders to collect sales tax, regardless of whether the business has a location in the state. Soon after the law went into effect, South Dakota sued online retailers Wayfair, Overstock and Newegg for allegedly failing to comply.

In a 1992 case involving mail-order office supply company Quill, the Supreme Court ruled that retailers can’t be forced to collect taxes in states where they don’t have a physical presence. But South Dakota argues that’s an outdated approach in an era when e-commerce accounts for an estimated 9.1 percent of all retail sales in the U.S.

A victory for South Dakota before the high court could open the door for more states to begin requiring previously exempt retailers to collect taxes, providing a potential revenue boost for cash-strapped governments like Illinois’.

Charging sales tax regardless of whether a business has a local presence is unpopular with retailers that still operate mostly online, like eBay and Etsy. They argue that it’s unreasonable to expect businesses to keep up with sales tax regulations in parts of the country where they’ll never set foot. Many want Congress to set up more standardized rules.

But retailers with more stores or distribution centers argue the exemption gives some online competitors an unfair advantage.

Viking Ski Shop, which has stores in Chicago and northwest suburban Barrington, will match online retailers’ prices on most items, but “we can’t give the tax away,” owner Bob Olson said. “Our profit margins are only a few percent, and at the end of the year, we don’t have room for that.”

“All I ever ask is to be on a level playing field, and we’re not when we have people advertising they can be 10 percent cheaper,” Olson said.

State and local governments, meanwhile, complain of lost revenue — an estimated $8 billion to $13 billion nationwide in 2017, according to the U.S. Government Accountability Office. That includes $383 million to $626 million in Illinois.

Shoppers should have been covering the difference as use tax — Line 23 on your Illinois tax return. But underreporting is an issue, experts say.

“The only people who pay use tax are those who think they may be appointed to a federal judgeship,” University of Illinois law professor Richard Kaplan said.

Seeing sales tax at more online stores could make consumers feel like they’re paying more, even if the tax isn’t new.

“I think there’s a basic American notion of fairness, that everyone should be subject to the same rules, but when people put on their consumer hat, there’s going to be some uproar,” Kaplan added.

As online retail continues to flourish, some bricks-and-mortar shops say having to collect sales taxes from shoppers puts them at a disadvantage.

Main Street retailers have a lot of challenges beyond sales tax, said Chuck Miller, president of Alpine Camera Co. in Des Plaines, Ill. Aside from the growth of e-commerce, his business has weathered the rise of digital photography, which took a toll on photo printing revenues, and smartphones, which hit the market for point-and-shoot cameras.

But sales tax is one more thing that can make it tough to match competitors’ prices, particularly with the sales volume of a mom-and-pop shop in an industry with thin margins. Alpine Camera has largely stopped selling new high-end digital cameras, which customers often could find online for less.

“[Customers would] say, ‘Let me go home and think about it,’ and what they’re thinking about is, if they buy it elsewhere, they’re saving,” Miller said.

Alpine Camera still sells used and vintage camera equipment, mostly on eBay, where Miller doesn’t have to collect sales tax from buyers outside Illinois. But it’s a relatively small share of the business, and sales tax is not likely to be the deciding factor for a customer seeking a particular vintage camera or lens in good condition.

“If I’m the only guy selling it and somebody wants it, I think they’ll pay the price,” he said.