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"The Internet's prevalence and power have changed the dynamics of the national economy", the court said in its decision in the case, South Dakota v. Wayfair. That was before the surge of online sales, and states have been trying since then to find constitutional ways to collect tax revenue from remote sellers into their state.
Currently, businesses shipping a product to another state where it does not have a "physical presence" - a store, office or warehouse - are not forced to collect that state's sales tax. Customers were generally responsible for paying the sales tax to the state themselves if they weren't charged it, but most didn't realize they owed it and few paid. Recently that has been expanding with States pushing to get sales taxes on sales even when the company does not have a physical presence in that area. It also impacts Amazon, but only for the goods it sells from third-party merchants.
In truth, this doesn't have much impact on Amazon, which has been collecting sales tax across the country since past year. The conservative chief justice, John Roberts dissented along with liberals Stephen Breyer, Sonia Sotomayor and Elena Kagan. Amazon only charges tax for purchases made directly from the online shop, but independent sellers using the site are not required to do so.
The case the court ruled in has to do with a law passed by South Dakota in 2016.
Other states will be doing the same thing and software and tax collection services will be gearing up.
Amazon's stock is down almost a percent since the announcement, Etsy is down 3.12 percent.
Some states will still have to pass new legislation to demand tax from out-of-state orders, and that isn't always guaranteed. First, the Act applies a safe harbor to those who transact only limited business in South Dakota.