He added that small businesses will face new burdens in trying to comply with a tangle of tax laws, giving examples.“Texas taxes sales of plain deodorant at 6.25 percent but imposes no tax on deodorant with antiperspirant,” Chief Justice Roberts wrote. “Illinois categorizes Twix and Snickers bars — chocolate-and-caramel confections usually displayed side-by-side in the candy aisle — as food and candy, respectively (Twix have flour; Snickers don’t), and taxes them differently.”

Source: Supreme Court Clears Way to Collect Sales Tax From Online Retailers – The New York Times

I suppose in practice you’ll have an API that is (upc, zip code)→(tax jurisdiction, rate)