Hi @coton61,
I build and maintain TaxJar’s plugin for WooCommerce. On behalf of TaxJar, I apologize for not proactively looking into your store to determine the root cause of additional API calculations after the first month you were charged more than expected.
We have seen caching and theme issues in the past that caused ongoing API calculation requests at checkout, potentially leading to overages. In order to debug this type of issue, we have to invest time thoroughly examining your store because each instance of WooCommerce has a unique theme and collection of plugins that can hook into the totals calculation process or affect how your store caches data. Recently we’ve made headway in reducing the likelihood of technical issues with our latest release v1.6.
If you decide to give TaxJar another chance in the future, I will work directly with you to make sure you no longer have to worry about API overages and can focus on running your business. Thank you!