Have any of you attempted to build a professional shopping cart and tried to get accurate sales tax working for states like California, Texas, and New York?

It's amazing how costly it can be because there's no easy way to map zip codes or any other easily looked up value to a tax rate. Zip codes can cross county lines and if a mall is built on a county line, there could be different sales tax rates within the same building. And yet, the states are no help in helping online stores to easily comply with the varying sales tax rates, even though they stand to make more money if people can more easily comply.

Even Paypal, Amazon Payments, Google, and other payment providers will not calculate sales tax for you, likely because it's so easy to get it wrong- The liability of miscalculating sales tax must be huge- Amazon has the money to fight the state tax offices but not a mom and pop online store.

There are several companies that exist solely to help shopping cart builders comply with the sales tax burdens of the different states, but the fees for using paid APIs can be high. One of these companies has map illustrating the problem.