Looking at the TaxJar code I believe the plugin does API call to TaxJar services to get rate by state and if necessary City and Zipcode and writes rate to nopcommerce tax tables?  Then cart usese nopcommerce tables to calculate tax like always.    If the rate is already in the tax tables does the plugin still do api call and rewrite nopcommerce tax tables for every order?   Please let me know if this is how plugin works?