Beginning Saturday, Amazon will charge sales tax to California residents thanks to a 2011 tax law that is about to go into effect. If you keep getting letters from the California State Board of Equalization asking for unpaid taxes on goods purchased online, you can blame it on Amazon. Citing a 1992 Supreme Court ruling that “prohibits states from forcing businesses without a physical presence in the area to collect sales tax” (via Hannah Dreier for techland), Amazon has gotten away with not collecting sales tax from California customers for years. What Amazon fails to mention at point-of-purchase is that you, the consumer, actually do owe those taxes—always have, always will. In leaving it up to the individual consumer to remember exactly what they bought, calculate the sales tax for it, and make out a check to tax gods, Amazon virutally guaranteed that hundreds of millions of dollars in taxes would simply not make it into California’s coffers.

The new law, which takes effect at midnight on Saturday, requires “any out of state retailer with a nexus (some sort of physical presence) in California” to start collecting tax. Some retailers, like Overstock, are still refusing to comply, but Amazon, which has many thousands of affiliates in California, has finally given in. You might remember the furor that arose when Amazon abruptly dropped all of its California affiliates in June of last year; the move, meant to pressure the California State Legislature and Jerry Brown into repealing the new tax law, ended up angering a lot of affiliates who had invested serious time in building their Amazon links, and who had helped the company drive revenue.

The new tax law is a big win not only for independent bookstores, but also for any brick-and-mortar store doing business in California. Bookstores have long been fighting a losing battle against the fire-breathing behemoth. (It’s no accident that Jeff Bezos named the company’s monopolizing e-reader the Kindle; Amazon has single-handedly lit the fire that threatens to turn all but the hardiest brick-and-mortar stores to ashes.) But even big box have been taking a hit from Amazon. From the smallest mom-and-pop hardware store (think the legendary Cliffs in San Francisco’s Castro district) to the “superstores” like Target, any store that invests in the community by leasing space and employing local workers has to charge sales tax. The fact that Amazon wasn’t collecting the tax–therefore further decreasing the cost of its product–has long made competition impossible.

It’s a strange case indeed that joins the interests of WalMart and the owner of a tiny bookstore in the Mission, but the tax brouhaha has done it. Last December, Amazon introduced a price scanning app and encouraged customers to go to their local bookstores, scan a book, and see how much cheaper they could get it on Amazon. Amazon uses brick-and-mortar stores of all sizes as showrooms for the merchandise it then sells online, without investing in the knowledgeable staff, the display space, and the community. Sure, you can grab a wrench from Amazon, but you don’t get to talk to the guy at the hardware store, who tells you exactly which wrench you need and how to use it. When I walk into Books Inc or Green Apple or the Booksmith or Kepler’s, I can count on someone who loves and knows books to recommend a great read.

Now, Amazon can no longer hide behind the “we don’t have a store here” loophole.  The online giant will be forced, kicking and screaming, to pay its fair share of taxes back to the state from which it profits so greatly. In an article for the San Francisco Chronicle, Andrew Ross explains the windfall of revenue the new law will create for California:

Besides shopping online with a clearer conscience, Californians should also see some benefit accruing to their cash-strapped state as it begins to claw back at least some of the estimated $1 billion in uncollected taxes, including $83 million from Amazon alone. State officials estimate that the new law should bring in more than $300 million in the first year alone, on sales taxes ranging from 7.25 percent to 9.75 percent, depending on the locality.

The court’s ruling will likely deal a serious blow to bookstores. Selling goods at below cost is inherently anticompetitive — only those with deep pockets can stay in the game — and Amazon has made a practice of targeting the very titles that sell best in bookstores for its predatory pricing. (Amazon recoups these losses in many ways, including by profiting on the sales of backlist titles that bookstores are unlikely to stock.)

Recently the Department of Justice filed suit against Apple and major publishers, alleging that they colluded to raise prices in the digital books market. While the claim sounds plausible on its face, the suit could wipe out the publishing industry as we know it, making it much harder for young authors to get published.