I feel sorry for you all with a tax system that is impossible to track.
The obvious solution is to ignore the fact that it is ecommerce and treat the sale as though the person was standing at the shop counter ordering goods - this is the idea behind ecommerce - a virtual shop.
This way the tax that should be applied is the tax of the state where the business is located. The sales channel should not effect the tax rate.
Back to my origional point - having differing state and regional tax rates is nuts. NZ introduced a flat tax (GST) on every item purchased or service rendered - no exceptions- some 15 years back - this is working well. Aus introduced a similar thing a few years back and it seems to be working- even though they added a whole bunch of exceptions that are exempt which increases the complexity of administratioon (AKA COST).
The US needs to bring its tax system in to the 1980's level of thinking (we wont ask you to advance too far:-) )
