Compliance is far more complex than a simple look up table. Rates vary by state, county, city, school district, mass transit zone, water district. And each of t

Compliance is far more complex than a simple look up table. Rates vary by state, county, city, school district, mass transit zone, water district. And each of these political subdivisions and the sales tax they assess changes with every ballot issue! There's a lot of faxing, forms to fill, etc. So most of the value is the vendor doing the legwork in keeping up with details and fixing corner cases. And leveraging their pre-existing trust network and economy of scale.