Also of note is that taxes would be calculated based on the shipping address which would prove beneficial in the end user knowing their correct total. Automagically.
That is the real trick these days and it will require a lot more than a simple plugin.
If you wanted a standalone plugin that managed all of this for you, you would need:
1. List of states by "origin rate" or "destination rate"
2. List of states in which the seller has physical presence
3. List of states in which the seller has sales tax nexus
4. Integration with the 30 or so individual state APIs for retrieving current sales tax rates, and a place to enter customer API keys for each- you must charge the current rate at time of sale, and rates change with some regularity.
5. Probably other things that I forgot, as well.
It's far too complicated for an small/medium business to actually calculate sales tax for out-of-state sales without help from a commercial third party. There are several third party services that you can purchase for this and a plugin for one or more of their APIs would be really fantastic.