[hakfoo]

This still requires vendors to correctly mark up their products-- to a level of detail that handles all the local rate differences, and shopping carts to be retooled to store said classifications and pass them to whatever tax-calculation service you need. Not to mention paying for the service to query.

It's probably less of a problem for someone starting from zero, but I'd imagine there's a type of vendor that's a huge nightmare. The firm that basically said "we can sell anything we can get from our vendor", and stocked their cart with thousands of SKUs, many of which exist only as lines in CSV files so they may not even know what they are offhand. The cart was probably built in the Eisenhower administration so good luck extending it.

It would be interesting to see some states offer a "trade convenience for savings" model-- rather than try to navigate a maze of regional rates and product categories to decide if a widget is taxed at 8.2% or 8.3, just file a one-page form and charge everyone 8.5% on everything. Saying "pay us $50 per year more in taxes, rather than spend $50k and ongoing service subscriptions to optimize the rates down to the penny" is a pretty compelling argument.