Cheap Taxes for Shopify is a Microsoft Windows application that allows you to calculate the taxes owed for each county.  It figures out which county each zip code belongs to.  Simply export your orders from Shopify, open the file in application, and it will automatically sum your taxable orders by county.  You can export the results to CSV, Excel, or Word.  There is no monthly fee for the application.  Buy it once and upgrades are free forever.