Well I would hope that different tax districts know what their tax rates are.  With modern software it isn't that hard to use a shipping address to connect to a tax district and auto populate the tax rates.  

One thing that these different tax districts and agencies could do to help simplify the process would be to have their rates easily accessible.  If a tax district needs to be paid I would hope they are making it as easy as possible to make sure the tax rates are correct.

What should be a simple percentage based on the shipping address has been made overly complicated when it shouldn't be.