Ya know... If Google really wanted to add a big heap of value for merchants regardless of cart system... They should figure out what other big online retailers (Apple comes to mind) are doing to get tax info for all jurisdictions. Then do similar to provide the correct tax rate for the rest of us with no extra setup. Keep it updated when rates change too.

So if the customer enters an address, if it's in one of the states that the merchant flagged as requiring sales tax, then the correct tax rate automatically appears...

That would be very cool.