Hyde here from Shopify. Great question!
Really the best and most comprehensive information for taxes in the US can be found here. The important takeaway is that taxes in the US are traditionally charged according to the state that you're based in:
You may actually, as a dropshipper, be classed as a "remote seller":
While Shopify sets tax rates by default for you based on your location, you should definitely consult with a tax consultant in your state to ensure that your tax rates are set up correctly.
I hope that all helps, happy to answer any other questions you may have.
All the best, Hyde.