Your'e talking about a max of 50 entities that will receive disbursements. Checking against a particular table to see the sales tax rate and then calculating it and adding it to a particular line item wouldn't be all that complicated.
I live in Texas (one of 50 states) we have a state sales tax.
I also live in Travis County(One of 254 counties in Texas) and Travis County has its own sales tax rate.
I do not live in, but do most of my shopping in Austin(one of 1,215 incorporated cities in Texas), and Austin has it's own tax rate.
Because Austin is Incorporated, things bought in Austin only have the state and city taxes, not the county taxes.
Looking at the state tax information page(http://www.window.state.tx.us/taxinfo/sales/) there are also 'Transit' and 'Special Purpose Districts' with their own tax rates.
Because I do not even know what sorts of Transit sales tax or Special purpose district sales tax might apply to me, I will ignore those for now.
This gives 1,470 independently managed tax rates in Texas alone. (Cities+Counties+state)
I would hardly be surprised is some of these taxing entities did not even have an online presence to contact about what the current tax rate is, and you can be certain that they will not all remember to send their updated tax information to every online retailer each time they tweak their local regulations.
Admittedly Texas is a pretty big state, and there are probably less than (1470*50=)73,500 distinct and independent taxing bodies (that can update their tax code either at any time they feel like it or according to a set of regulations that are probably distinct for each one) in the United states, but that is at least the correct scale of the problem.
Or it is if you *only* sell to the United States, if you sell international too, things start to get messy...
Do you even know if the city but not county rule/law is in effect for all the states in the US?
I have lived in different parts of Travis County for more than 30 years, and I do not even know if there are any Transit or Special District sales taxes that I currently pay, and you think that some start-up that sells novelty bobble-headed dolls and pez dispensers could easily track down all the taxes they need to charge each and every one of their customers?