[pitaa]

Unfortunately the zip code level is not always precise enough to determine local tax rates, but your point stands that it isn't usually overly difficult to find the tax rates for a particular shipping address.

I remember a few years back when I was doing ecommerce, I came across a service called TaxCloud[0] that was supposed to handle all of the tax collection for you, for free. I never actually used them, but the idea was you use their API to calculate tax rates for all your orders, you remit the tax to them and then they paid the states. Apparently many states offer a commission to 3rd parties that collect and remit owed taxes, so TaxCloud was supposed to be making their money that way.

[0] taxcloud.net