Seven years and one U.S. Supreme Court decision later, Colorado’s law aimed at prodding online retailers to collect sales taxes took effect this month.

Better known as the “Amazon tax” law, it actually doesn’t change a thing for the dot-com behemoth, because Amazon has been collecting sales taxes in the state since February 2016.

But it gave other large online retailers an uncomfortable choice: Start collecting sales taxes now, or pass that responsibility on, leaving their customers with a tax burden later.

Here’s a primer on the law’s origins and how it could affect you.

Wait, so what is this law, and why was it created?

The law is designed to close a tax loophole created by the boom in online retail. In Quill v. North Dakota, the U.S. Supreme Court in 1992 ruled that customers do owe state sales taxes for online purchases, but states can’t force companies to collect it.

That’s been a boon to online shoppers, since states typically don’t prosecute residents for failing to file their own taxes on online goods. It’s also given online retailers a competitive advantage over businesses with a brick-and-mortar store in Colorado. And, it’s cut deeply into state and local government revenues. In 2012 — before Amazon started collecting sales taxes — Colorado was losing an estimated $350 million a year in taxes on online sales.

To fix it, the Colorado legislature came up with a workaround that’s become something of a national model: Force retailers that don’t collect sales taxes to file reports on how much their Colorado customers are spending and warn customers that they may owe taxes.

What it means for online retailers

Chiefly, more paperwork — assuming retailers don’t simply give in and collect sales taxes, instead.

Large online sellers that don’t collect are now required to send customers a notice every time they buy something explaining that they may owe what’s called a use tax — essentially, sales tax that wasn’t collected at the time of purchase.

If a customer spends more than $500 in a year, the retailer has to send the customer an annual summary of their purchases so shoppers have the information when filing their tax returns.

Finally, companies will have to file an annual report with the state detailing the following information for each of their customers: their name, billing and shipping addresses, and the total amount each person spent with the store that year. What it won’t say is what exactly the customer bought — state law explicitly prohibits retailers from reporting itemized purchases.

These new regulations apply to any company that sells more than $100,000 worth of merchandise a year to Coloradans but doesn’t collect sales taxes from its customers.

What it means for online shoppers

If you spend more than $500 with a large online retailer that doesn’t collect sales taxes, you’re supposed to receive the aforementioned notice at the end of the year detailing what you spent.

And if you buy anything at all, you’ll get the initial reminder that you may owe the state money.

As for the taxes you may owe, nothing technically changes there — the state will just have records detailing what you owe. Online shoppers in Colorado have always owed use taxes when retailers refuse to collect sales tax.

But few people actually paid it, according to state finance officials. A Colorado Legislative Council fiscal analysis found that, in 2015, just 79,000 of the state’s 2.5 million income tax filers paid such taxes, even though as many as 8 in 10 people shop online.

Use tax is due annually on April 15th, much like income tax returns. Contact your tax preparer or visit the Department of Revenue website for the proper form.

All of the purchase information the state collects — much like your tax returns — is confidential under state law.

What it means for brick-and-mortar shops

Nothing — the law doesn’t apply to retailers with a physical presence in the state, because they’re already required to collect sales taxes.

But proponents of the law hope it will nudge more online competitors to start charging sales taxes, putting them on a more level playing field with more traditional stores.

What it means for the government

The legislative fiscal analysis estimated the state of Colorado could collect $5.9 million in additional sales taxes this year with the law in place. Local governments, too, are likely to see sales tax revenues grow.