After a very long development cycle, we now have custom modules for Zen-cart and Oscommerce that can calculate destination-based sales tax. Using a combination of address verification and data made available by your State you can become compliant in no time flat.
The modules calculate based on Zip +4 to break down the sales tax by zip code and the 4 digit code as well. If you tax rates change quarterly like our state, you can upload and parse the CSV file they make available to you in order to update the rates in bulk. For more information, contact us here.