Hey @majornorth ,
I hope your business is going well. I realize this your post was from a year ago, but I thought this updated chart on economic nexus thresholds would help you and other sellers. Some states have removed the 200 transactions, but there are still 27 states that once you pass 200 transactions, you have created nexus https://salestaxsystem.s3.amazonaws.com/Sales_Tax_Compliance+_Economic_Nexus_Shopify.pdf.
In regards to the dropshipping, that is where exemption certificates will come into play from your supplier. We are working on a new PowerPoint and video soon that will simplify this complicated part of the dropshipping equation.
I hope this helps.
Kind regards,
Scott Letourneau
Shopify Tax Partner