Unless you already signed up, use something else - the system is complicated and it will cost you a lot. Keep an eye on API calls as NOP is most likely paid by Avelara for each customer they bring in, and Avetax makes $$ on API calls, so they want many API calls for each order.
Manoj at Execula can help you rewrite the plugin so it only makes one call / order. [email protected]
TaxJar works well for me, Jatin can help you [email protected]