When I was doing a lot of ecommerce we used to recommend Avalara (www.avalara.com), for both US and international businesses in regards to sales/use tax compliance and import duties.

They are not a SaaS startup where you just sign up and use it, but for all the headaches they took off clients hands they were worth it. I don't remember their costs, and it has been long enough now it probably wouldn't be valid anyway. Not sure they'd be cost effective for a smaller business but I'd call them if I had a need.