Don't you just love taxes?
Here's a tricky scenario for you.
Minnesota and Wisconsin.  I have to collect sales tax in both.  Minnesota taxes can be weird; for example, if you live within x miles of the stadium in Minneapolis, there is an additional 1/2% stadium tax.  Well, it's not a different zip code, so there's really no way to tell.  So anyone in that town gets charged the higher tax.  Sorry.
Wisconsin is by COUNTY.  I have previously used a huge lookup table for that; one with every city/town in wisconsin, and what county they are in.
But here comes the really fun bit.
Clothing.
Clothing is not taxed in Minnesota, unless the article of clothing is > $100.
Clothing *is* taxed in Wisconsin.
So, I can't check "tax exempt", because that doesn't work for Wisconsin.
I can't tax all items, because that doesn't work for Minnesota, which taxes non-clothing items.
Using a service such as a zipcode to tax lookup doesn't work; because (for example) Duluth and Hermantown have the same zipcode, but two different taxes.
Did I mention that I *really* hate tax laws?
So... meanwhile... I've been using a huge tax lookup table with some very custom SQL Scripting (Oh, yes, we also have to collect taxes as well as VAT and duties for anything going to Canada)...

So, anyone have any thoughts on how to make something conditionally taxable (Tax it if it's in a specific state, but don't tax it in any other state)