For states, the trick is to get their tax collection system up to snuff - unifying sales and use tax collection to the state tax commission/department, making it easier to figure out local tax rates, and providing consistent and regular updates on rate information.

I think the out of state tax rate should be whatever the base rate for the state is. In California, nearly every county has its own rate in addition to the base sales tax, and it’s not unusual for me to be charged the highest tax rate in the state instead of my local one. It would simplify things for everyone if there was only a single tax rate charged on out of state purchases.